Yes, that's it.
I'm not certain of the answer on that, but it would depend on balancing how soon it is possible to get good forecasts of demand and the planning horizon for fixing supply of rolling stock and traincrew resources. I wouldn't want to go as far as on-call working, but there might be scope for having fewer rest days at busy times and more rest days at quiet times.
And where the railway is now, it is the money that talks.
Yield management wouldn't go away, if anything it would become more important. Yield management would still be needed to avoid running trains with 5 passengers, and it would still be needed to ensure that rolling stock and traincrew were not idle. But it has to be a step forward to be able to manage supply and demand not just demand.
I'm glad someone has mentioned football. Flexibility of supply could be a significant benefit on football matchdays. For example, if Liverpool are kicking off at 1230 and Manchester United at 1500 then early morning capacity from London can be weighted towards Liverpool and later morning capacity towards Manchester. Similar considerations could apply to other big sporting and cultural events.
There's a huge amount of data on ticketing and travelling now, and the computer power to process it. That opens up potential for the railway to manage demand and supply in ways that weren't possible 20-30 years ago, driving revenue up and costs down. Yield management has been doing that on the demand side for a while now. The supply side is more of a challenge, but every crisis is an opportunity, and the railway is in crisis now.
This has strayed from my original point, which was that, when most tickets are advance booked, 3 trains per hour do not need be every 20 minutes.
I am all in favour of reforming timetables - clearly, the current model offers a poor utilisation of the resources that are made available to the industry.
However, I fail to see how your proposals deliver an improvement to passengers. I agree that it is frustating having trains every 20 minutes if the price of a flexible ticket is so high that you realistically have to tie yourself to one train with an Advance.
But this is something which can be addressed by changing the ticketing - firstly, by allowing Advances to be quickly and easily changed almost up until the moment of departure, perhaps with a stepped fee depending on how close to departure you make the change; secondly, by reducing the ludicrously overpriced level of Anytime fares on long-distance routes. The ratio of Anytime to Off-Peak fares should never be more than 2x, and in most cases lower than that.
Having trains at frequent intervals also reduces the Generalised Journey Time, which is important in ensuring the competitiveness of trains compared to the car. It means that connections between trains, or between trains and buses/Underground/light rail are likely to be shorter. So even in a model where nearly everyone is forced onto Advances, it still has some value. For instance, on a route where there are flights once every 2 hours vs two flights within 10 minutes then a 3h50m gap, the former is much more likely to be convenient for my plans - even though I'm tied to the flight I book.
What you suggest would also be impractical on many routes, such as those with mixed calling patterns (would all trains have to be slowed down to stop at all stations?), those with mixed routes (what would serve Wilmslow if you only had Manchester trains via Macclesfield?) and those where the number of platforms simply isn't high enough to have loads of trains sitting around at the same time (this applies almost everywhere!).
Flighting is also already used on many mainlines, but typically with trains to different destinations and calling patterns following each other. For instance, on the GWML you can have trains departing Paddington at 2 minute intervals heading to Taunton, Plymouth and Hereford. What would the Plymouth and Hereford services do if you had to allow 'shadow paths' for possible relief services to Taunton/Bristol? Would you rewrite the entire timetable on the lines those services head to, just to allow this new structure?
Like I say, I am all for change, but I just don't think that this would be the answer.