Interesting to compare with Hereford. Both county towns a similar distance from London. Shrewsbury (with its 40% larger population) sees 20% more London passengers than Hereford does and yet the latter has four times the number of direct departures. Why is Avanti's Shrewsbury service deemed uneconomic when GWR's Hereford services aren't?
I do wonder if the fact that there are more journeys between Hereford and London per capita than between Shrewsbury and London is an illustration of the effect that having a direct service has on demand.
You do realise that the early morning peak service from Hereford towards London has been reduced from three trains down to one, don't you? And four evening arrivals is now two. Hardly suggestive of great demand there either, is it?
The survival of that one morning service to London probably has as least as much to do with there only being three shoreline-equipped sidings for IETs at Worcester Shrub Hill and an extra one being available at Hereford, than it does with the number of passengers using the remaining service from Hereford at 05.23. At the same time, the off-peak runs to Hereford help to keep the hourly timetable on the Cotswold Line balanced. If the infrastructure at the Birmingham end is eventually provided to allow an extra service each hour to/from Hereford, then pathing west of Malvern Wells is going to be an issue, as several of the GWR services fit into the gaps between WMR trains.
Great Malvern itself seeing fewer London passengers than Telford does. Worcester Shrub Hill has a similar number again, the trains don't get properly busy until Oxford.
Of course the main difference is that the London-Worcester service is not in addition to stopping services, it IS the stopping service. The fact that it's operated by Intercity rolling stock is a consequence of our DMU shortage, it used to be mostly Turbos. If Shrewsbury is ever to sustain a direct London service, it would have to be as an extension of the Aberystwyth - International trains. That way it's not duplicating an existing service. Doing so would require more joined-up thinking than the fragmented railway allows for, but at least a path would be readily available once HS2 has taken some of the existing Birmingham traffic.
The trains don't get properly busy until Oxford? That will come as news to passengers in the Cotswolds on many services.
Dr Fegg has already pointed out that your claim about use of IETs because of dmu shortages is nonsense. IETs on the Cotswold Line were part of the GWR game plan for the fleet from the start, because the Turbos were inadequate for the traffic much of the day - and had been even 20+ years ago - hence the use of 180s from 2004. What would things be like in terms of dmus for the Bristol area if the IETs had not taken over all but one Turbo duty on the Cotswold Line?
No it isn't. It's a consequence of FGW seeing potential for revenue growth in the Cotswold market, and investing in improving service provision. The move to Intercity-quality rolling stock for (most) Cotswold Line trains dates back to the First Great Western Link era, before the DMU shortage.
From
https://en.wikipedia.org/wiki/First_Great_Western_Link:
Though we could of course trace the whole process of improving Cotswold Line rolling stock back another decade, to 1993 and the reinstatement of day round through running to Paddington alongside the introduction of the Turbos, which sparked the first stage in the growth of passenger traffic on the line, and was a clear step up from the West Midlands area Class 150s previously used between Worcestershire and Oxford.
It was operated by turbos because of a HST shortage, the Cotswold and Oxford services were quick to get bumped to a turbo to ensure that South Wales, Penzance etc. could get a HST.
Cotswold Line has a very different flow to Shrewsbury. For example Charlbury's top 5 in the destination matrix are below, it has a lot of London journeys. Given the people the Cotswolds attract (such as David Cameron), it isn't suprising that it has good traffic to London, so keeping the stopper attached to an Oxford - London makes sense.
Meanwhile for Shrewsbury the main flow is to Birmingham. The numbers to London Euston are quite good at 50000 (average 136 a day) but this is a terminus station.
No one was going to provide HSTs on everything on the Cotswold Line, ever. FGW upped the number it used in 2009 after the first departure of the 180s from GW services, but soon pulled back, as it was costing tham a packet in extra operating costs. And only one or two Cotswold services were allowed to be targeted for HSTs to be pinched on weekdays, notably the 17.50 Paddington to Worcester.
David Cameron is hardly representative of the local population - and the area's main attraction for him was a nice safe seat in the House of Commons, not the train service, though he did lobby hard for the redoubling project.
Entirely agree re Shrewsbury/Shropshire traffic flows - even if some people are splitting tickets in Birmingham on journeys to/from London, the fact remains that Brum is the place most people in Shropshire are travelling to, not London. BR knew it at the start of the 1990s, Virgin soon found out a few yeas later, at some expense, that BR knew what it was talking about and the recent Virgin/Avanti London services were as much to do with politics as providing travel options.