On this thread generally...while this is in the Not Happening(tm) bracket, even if a Serpell style proposal was made, I don't think it would make travel impossible. With greater use of bus and coach lanes on urban roads and motorways, and a network of motorway Coachways (think MK or Handy Cross), bus and coach could provide for the UK's public transport needs, plus air if you need to be there quick.
It'd not be great, though, it's better with the railway, and the effect of that would be hugely more cars on the road.
Indeed, whilst some would switch from rail to coaches/buses (which would be extra vehicles on the roads anyway) there would be a fair chunk who would switch to cars.
Given how busy GWR services get at peak times, adding an extra 200 cars an hour using the A303 is going to add to the traffic congestion (as the peak GWR times also the to be times when the A303 are the busiest).
Whilst not everyone who uses the GWR services to/through Exeter would then use the A303, although probably enough to may that sort of difference.
It would meaning that all the missing sections of duel carriageways would need to be built to reduce congestion, probably with some of the roundabouts needing to be upgraded.
Of course they would only solve part of the issues which GWR services solve. For instance the trains to St Ives can be very busy and there's not the ability to park that many cars in the town, whilst any buses would be held up in the traffic.
Whilst GWR may be able to be closed down (at a cost) and it be viable (at a cost which was acceptable), if this was repeated across the whole country there would be a lot of places which would need fixing, probably quite quickly (read at quite an expense).
However for any places which are provided with bus services which needed any sort of support, how trusting would we really have that this support would continue?
I for one (especially given the level of cuts over the last 12 years) would be very sceptical that it would be protected. Especially given the level of road building which would also be required.