Why do you think people might want to replace a cramped regional-railways-style train with an InterCity-style train on routes of 3 hours and more, some of which are more than 5 hours long? Comfort, luggage space and buffet to name but three.
125mph Voyagers would probably be wasted on many of the routes, plus Stansted is a problem, but Turbostars are not ideal suits for the service either.
I'm not sure what journeys the Turbostars do that are "more than five hours long" (?) but don't think that the luggage capacity on Voyagers is one of their main selling points - I don't mind them for comfort but some of our more "sensitive" members seem to hate Voyagers for comfort
I've always wondered why XC have services which seem to reverse at New Street? Newcastle to Reading and Manchester to Bristol specifically. Wouldn't it make more sense to just have 2 tph from Manchester to Reading and 2 tph from Newcastle to Bristol?
I think that XC would have been a lot better if that was how things operated (back in Virgin days, it was 3tp2h from Newcastle to Bristol and 3tp2h from Manchester to Reading with each "core" section being half hourly when combined with a bi-hourly Newcastle - Reading, bi-hourly Scotland - Wigan - Bristol and bi-hourly Manchester - Birmingham terminator)
If the two "sides" were split like this (to give a half hourly York - Bristol corridor and half hourly Manchester - Reading corridor, with various extensions beyond etc) then you could make improvements to each section without the mess of everything being linked (e.g. pre-Covid the Bournemouth - Manchester services ran onto Manchester - Bristol services and vice versa, so everything was tied together) - if they were separate then you could dedicate the EMR 222s to one "side", or have a better business case for something like an 802 on the Manchester - Reading corridor (since it'd spent a large proportion of its time under the wires - whereas if you continue to interwork them with Bristol services plus run Newcastle - Reading trains too then you'd need to order a lot more bi-mode trains which makes the whole thing unaffordable)
You could also look at speeding up one "side" of XC, since the York - Bristol corridor could be sped up without being tied to paths from Birmingham to Manchester/Reading
Obviously it'll annoy people wanting direct links between various cities hundreds of miles apart but I honestly don't think that there are a lot of long distance links that
need direct trains (other than ones involving London/ Birmingham) - e.g. Sheffield's long established links to places like Plymouth are nice to have but not essential, it'd be no skin off my nose if Sheffield services went to Cardiff or somewhere else instead