That was kind of the point I was trying to make - the Cardiff-Portsmouth route already has stock with no end gangways so class 175s (also with no end gangways) would be ok. However, the Castle HSTs are 4 carriages and gangwayed throughout, so in that respect it's better to replace the Castle HSTs with class 158s (thus preserving through gangways) than with class 175s.
Many EMR InterCity services do skip Loughborough, and the ones that do stop are operated by the same (end-door) units as the ones that don't. Therefore keeping both shouldn't be an issue but if stopping at both Cosham and Fareham with a 5-car class 175 formation (or equivilant) causes a problem with dwell times on the Cardiff-Portsmouth then drop one of them* to allow end-door stock to be used - there are plenty of Southern and SWR services there to serve them.
* I would suggest dropping both except that they are useful for interchange with services towards Havant and Hedge End so at least one of them should remain as a call on the Cardiff-Portsmouth for that reason. Returning to your EMR example, I feel that dropping Bedford calls from the Market Harborough services was a mistake due to the lost potential for interchange with East West Rail.
The reduction in changes on some flows is a happy side effect, but looking at the specification of the rolling stock (Crossrail even more so than Thameslink) that is clearly not their primary purpose. For example if I was going from Reading to Norwich then (at least if the IET's had decent seats) the way to go would clearly be GWR to Paddington then Crossrail to get through London and then GA to Norwich, despite the fact it could be done with one change if using Crossrail from Reading.
So? I'm saying the almost exact same thing but for a different subset of passengers. The Cardiff-Portsmouth (and other similar routes) is also the only service that can take them from end-to-end, so it is just as vital for long-distance passengers as you're saying it is for commuters. In other words, it is
both a series of commuter route(s) and a long-distance route.
A short-distance commuter isn't going to care if they have a nice big table, extra legroom, a clear view out of the window etc. - they just want to get to work/home and might not even mind having to sit on the floor (or stand). However, a long-distance passenger (particularly if travelling for leisure) does care so those 'quality features' need to be there for them and giving away space that could have been used for legroom/tables/toilets/seats by having wide doors doesn't help. Having wide doors cuts dwell times, but it almost certainly means fewer seats, tables and/or toilets. That increases the risk of the long-distance passenger having an unpleasant journey and vowing never to travel by rail (at least on that route) again. Just don't take that risk, make sure the long-distance passengers have access to class 175s (or equivilent/better).
Short version:
a short-distance commuter doesn't care if he/she gets a 166 or a 175, a long-distance passenger does.
If you are giving GWR (semi-)fast Bristol-Exeter services (direct services
avoiding Weston-Super-Mare), plus the 175s
and the 222s then I would do something similar:
- Bristol-Exeter (direct services avoiding Weston-Super-Mare) - class 222s (agreed with you)
- Exeter-Penzance - class 222s (agreed with you) or class 158s
- Cardiff-Taunton/Exeter (stopping services via Weston-Super-Mare) - class 166s/165s (disagreement)
- Cardiff-Portsmouth - class 175s (in 5-car/6-car formations)