As someone who's a bit sceptical about the project, it boils down to a couple of issues for me:
I'm sceptical that future Governments will stick to the stated policy of HS2 spending being considered separately from spending on the rest of the railway, particularly when decisions are made behind closed doors.
The effect of HS services on existing main lines - having seen the South Eastern main line turned into a slowed down all stations backwater to make people pay a premium for HS1 (although hopefully this may be alleviated in the near future).
Currently the spending on HS2 appears to have done what we expected it to, which is to switch the Crossrail spend to HS2. As such, for now spending on the rest of the network appears to protected. Whether that will continue is another matter, but what I will say is that with all the political interest in rail of late it's going to be politically difficult to reduce rail spend.
I would suggest that Birmingham to London is different to the Kent services. Firstly if the existing services slow down then more people would switch to Chiltern from Birmingham. From many of the other stations chances are they'll still retain mostly the existing calling pattern (maybe plus one or two stops) but then gain some more frequent services. It's worth noting that links like Preston to Coventry would start to be where the existing services would do well post HS2.
I would guess that:
- Scotland/London services would all be run via Birmingham (with the faster journey times being achieved by changing at Crewe) with an extra HS2 service added to increase frequencies (and further increase the journey time saving). With the current fast service having stops at Crewe, Birmingham (NS and International), Coventry and Rugby.
- The two Birmingham services would call at Rugby, MK and Watford Junction, possibly with a more regular service via Northampton. As I doubt that the other stations would generate enough passengers to justify the extra time required for the stops and possibly switching to the slower lines, especially as they could gain other services from other paths being freed up.
- The Manchester services having one path a path for local services in Manchester and London and the other having additional stops added so it calls at more stations in the Trent Valley, whilst still retaining the route via Birmingham unchanged.
- The Liverpool and Chester services being services that run along HS2 to Crewe and then use the existing lines. With the paths at the London end being used either as a LM style service to Crewe or for local services.
They would mean that Coventry would see it's services change from 3 trains or hour with one stop to 4 trains per hour with 1 I've stop service, 1 two stop service and 2 three stop services. That would mean that at worst there'd be 6 minutes added to a journey, but there's still two services which are the same or only 3 minutes slower. However it then gains two trains per hour to Crewe and onwards North and more trains to MK, Rugby and Watford Junction.
MK would gain better connections to Birmingham, Rugby and Coventry.
Manchester would gain better connections with the Trent Valley, plus a local service (maybe more as there'd be fewer 390's blocking platforms whilst they were turned around ready for their next service).
The Trent Valley would see one, maybe two, trains calling at its stations.
London would gain at least two (maybe 3) local services, possibly one being like the existing Tring services but extended to MK and one being an all stops service much closer in.
All the stations would gain more capacity on many of their trains as people switched from the 390's to the new HS2 services.
That should result in a fairly balanced overall resulting timetable, with there still being hourly headline fast services but with extra connectivity on the other services. With few passengers missing out, with the exception of those unwilling to change trains at Crewe.