To be honest, I can^t see bi-modes being used on Services to the Southwest for the foreseeable future, not until more of the route is electrified.
In short I can see
HSTs▸ being retained for Southwest Services ^ and, in this context, by Southwest services I mean Pad ^ Westbury/Frome/Taunton/Exeter/Paignton/Plymouth/Newquay/Penzance services. (For the
current summer timetable this would require about 20 HST sets)
Plus with the current political backtracking I doubt that there will barely be enough bi-modes to work Pad ^ Worc/Great Malvern/Hereford/Cheltenham/Weston^super-Mare/Carmarthen/Pembroke Dock Services.
Another problem is that bi-modes, if they^re limited to 100mph, they cannot reach the 110mph line speed on the Newbury ^ Westbury route and the Uphill Junction ^ Taunton route, therefore I cannot see them being introduced until a time when more of the route is electrified.
In any case, with some exceptions, total fleet replacement is not as good an idea as partial fleet replacement. Partial fleet replacement means that you have plenty of spares for the remaining fleet. Therefore I don^t think that leaving HSTs on West Country services is such a bad thing (plus all the other advantages an HST has).
I remember the last years of loco-hauled on the Exeter ^ Waterloo line where everything was seemingly deliberately run down, partly to make the new units look better. Good locos were withdrawn and sold and locos in poorer condition were left to carry on with not enough maintenance. Also more modern Air con stock was deliberately kept off the line, despite being available, (
with the exception of first class vehicles mind you, it was ok to upgrade first class vehicles to air con stock!). I also seem to remember the micro-buffets disappearing from this route around the same time.
One final note about buffets is that they^re useful normally and a god-send when a train is delayed. If you get stuck on a train somewhere, knowing that you can walk down to the buffet for refreshments makes a lot of difference.