I would go for 158 and 159 replacement being with cascaded 220s, 221s & 222s. Those in turn cascaded from their present routes by 80Xs once more electrification has happened. But I can only dream.
No, just no! Have you ever bothered to look how much higher voyager floors are than 158/165 units or many emu types?
With the low platforms at quite a few stations along the route, the fit and able would struggle to board the trains let alone anyone with poor mobility or encumbered with luggage, or young children.
If you want a suitable replacement for 158 / 165 on Portsmouth - Cardiff an ac-3rd rail-diesel / battery/ hydrogen / unicorn poo version of the 745's in East Anglia would be ideal. Wide doors, 2+2 seating, good performance and brand new also capable of running on electricity between Cardiff and Bristol and Southampton and Portsmouth/Brighton.
When withdrawn there is only two solutions for the voyagers - replacing rusted out pacer coaches in schools/ hospitals/club houses and your local neighbour hood scrap merchant.
Push-pull loco-hauled operation avoids the need for run-round at terminals. Class 365 sets might be be suitable candidates as they have 2+2 seating.
Nearly back to the future then.
In 1975/1976 the Southern Region wanted to take over the whole Portsmouth Bristol service and run it with class 33/1's and 4TC units. Unfortunately there wasn't enough of either type and from what I remember of the conversations I heard a few years later the BRB would not consider converting any more class 33's nor rebuilding mark 1's into TC units.
WE got second best. Hand me down mark 1 steam heat stock and completely knackered class 31's. Things did improve with eth stock and former Finsbury Park 31/4's. Reliability was only gained with the allocation of class 33's to the route.