The trouble with compartments is that they provide a considerably lower average "seat quality index", if I may so term it, per group of seats than opens do.
Consider a second-class compartment vs. a corresponding group of seats in an open - both of which provide 8 seats per window's length of the carriage (since when compartments were common they hadn't yet forgotten how to align seats with windows in opens; and assuming the armrests are up in the compartment).
With the compartment, you get only one decent seat out of 8 - ie. next to the window, facing forwards (or backwards, come to that, if that's your preference); one not-so-decent seat (next to the window but facing the other way); one tolerable seat (next to the corridor facing the right way; not a great view but at least it's something); one annoying last-choice seat (next to the corridor but facing the wrong way); and four rotten seats which are barely any better than sitting on your suitcase in the vestibule (strangers sat on both sides of you and nothing to look at except the wall around the head of the person sitting opposite; these seats suck).
With the open, you get two decent seats by the same standard as the above, two similarly not-so-decent seats, and four seats which are a bit crap but still significantly better than the four rotten seats in a compartment, as you at least have the aisle space on one side of you instead of people on both sides, and a much wider choice of places to point your eyes at.
I had some most enjoyably cosy trips fugging up in a Mk1 compartment on a nearly empty train, but for a train that was even merely lightly loaded as opposed to nearly empty, a rake of opens gave a much better chance of getting a good seat.
Compartments are not precluded by the deplorable desire to cram as many people in as possible, since (as described) both compartments and opens give you the same capacity of 8 seats per window's length - the difference in that respect is only in whether the passage for people to move up and down the train is in the middle or at the side. (I am assuming that "Inter-City standard" precludes consideration of 2+3 open seating.) But they do feel a lot more crammed-in on an equally busy service because of the four seats which are thoroughly rotten rather than just not very good.
The rot with cramming people in began with the introduction of HSTs in place of rakes of conventional hauled stock that were several carriages longer; this contributed to HSTs being the trains to avoid on a route served by both HSTs and conventional stock by reason of always being significantly more crowded. It became a real problem a few years later, when skinflinting on the orders for new DMUs led to two-car and even single-car units replacing not only the two-car DMUs common in some areas, but the three-car DMUs common in other areas and the rakes of 4 or 5 Mk1s that yet other areas liked to use in place of DMUs on some services - and most unfortunately they got away with it because there was not any massive surge of complaints from passengers about the missing carriages. That then set the mood for ignoring any such complaints that they did get, and we got to suffer from more egregious instances of the same trend such as cross-country services, already reduced in capacity with the change from conventional stock to HSTs, being thoroughly rogered by the replacement of HSTs with stunted Voyagers of half the length.
We now seem to have effectively become "locked-in" to the stunted formation anti-pattern mainly through multiple manifestations of obsessive adherence to the counterproductive principle that when people have adopted a silly habit of doing something in an awkward and obstructive manner that gives rise to multiple secondary difficulties in place of the former straightforward and functional practice that did not cause such difficulties, the awkward practice must now be regarded as absolutely set in stone as if it had been established forever, no matter how daft it is, how problematical its results, or how recent its introduction, and defended to the hilt with the fervour of a 17th-century evangelist, with any suggestion of not doing things like that and adopting alternative habits which don't tie everything else up in knots being regarded as heretically anathematical and deserving of burning at the stake. A particularly pernicious instance of this is that we have allowed our legal system to become crippled by the debilitating disease, apparently an infection of US origin, of assuming that if anyone undergoes any kind of misfortune then (a) it can't possibly be their own fault no matter how clearly it actually is, and (b) it is therefore a valid reason for screwing some free money out of whichever legally-defenceless poor sod can be lumbered with inappropriate blame for someone else's stupidity.
I used to travel regularly from Paddington to Worcester on services formed of about 12 carriages which once they got beyond Oxford would make several stops apparently in the middle of nowhere going by the view through the window; but on lowering the droplight and peering along the length of the train it could be seen that two or three carriages were in fact aligned with a tiny little station platform, with a considerable crowd of people streaming out of them. And this was never a problem. Nobody was ever such a complete and utter fool as to step out of one of the doors that was not aligned with the platform and fall into space, and if they had they would simply have been told to look where they're flipping well going in future - a lesson so basic that any animal understands its importance as soon as it is old enough to walk. Contrast that situation with the mess we are in now whereby someone equally dismissive of the fundamental requirements of existing as a creature capable of independent movement is able to abuse the legal system to screw a pile of undeserved money out of the railways for not physically restraining them from being a blithering idiot - and nobody seems interested in making the slightest effort to correct this ridiculous situation (maybe everyone's waiting on the day when they get to do it themselves...) The slavish acceptance of this specific state of an inherently mutable set of conditions as if it was as permanent and unchangeable as a law of physics resulted in the provision of capacity to those small but heavily-used stations being reduced from the former appropriately long formations to whatever stunted DMU would not be longer than the platforms, so for this entirely artificial reason the service was condemned to a lower standard of quality for far too many years until the pressure to improve it eventually outweighed the whining about how much it would cost to extend the platforms (albeit the figure was indeed ridiculous for comparably daft and not unrelated reasons).
Other inappropriately-maintained tenets in a different area include the notion that everything has to be a multiple unit with every vehicle powered, so every vehicle is a complex piece of machinery with a complete set of powertrain equipment that costs the complete amount to build and to maintain, instead of the majority of them being simple boxes on wheels needing nothing more complex than brakes, lights and heating, which makes longer trains disproportionately more expensive and fosters the "cram 'em in" attitude; also the notion that every vehicle has to be used as close to absolutely continuously, and crammed as full as possible, all the time as can possibly be managed, with the idea of having some of them spare for dealing with peaks of demand and covering for contingencies being regarded with screaming horror, so services at times of peak demand become sardine cans formed of the same few vehicles that can be filled with the lesser demand at other times (or alternatively in a few cases services at times of low demand get run with fully-peak-sized trains instead of leaving the surplus vehicles at the depot, never mind the extra maintenance and fuel costs, which is arguably even dafter but at least isn't detrimental to passenger comfort and crowding).
Then there is the closely-related practice of assuming that whatever precisely specific level of traffic happens to be what there is right now is the exact level that will always and only need to be handled now and forever after, which invariably ends up causing a disproportionate amount of trouble and expense some years in the future when the assumption is shown to be invalid; examples of difficulties thus engendered on the railway system as it is now are too numerous to mention, but the particular one which is relevant here - at least according to what I have made out from many comments on this forum - is that recent resignalling programmes have made the assumption that trains are never ever going to be any longer than the maximum length that happens to be the case at the moment, so any idea of introducing longer trains is clobbered before it can get started by the need to do the resignalling all over again. With that sort of short-sighted imposition to cope with it's difficult to see how not to have to resort to cramming 'em in.
The Midland had the right idea: OK, so our route to Scotland may not be as good as the alternatives, so we'll compensate by making our services the most comfortable and giving the pleasantest trip. If the desire to attract people to using trains instead of cars is to be taken at all seriously, it is of great importance to pay attention to making the trains comfortable. If you have a car it is almost always more convenient to drive all the way than to catch a train (taking cases like going to London as exceptional), as well as being considerably cheaper, so the train is facing a serious disadvantage from the start; on the other hand, few cars, no matter how comfortable they may be for short periods, remain comfortable throughout a long ("Inter-City") journey, whereas a well-designed train can remain comfortable for such journeys. So it is necessary to maximise this advantage, and not throw it away - in particular not to throw it away through following the "cram 'em in" ideology, which leads to particularly unpleasant journeys and makes your own car a much more attractive option.
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