Who knows. I can see the first two happening but I can't see through-ticketing disappearing. Rail needs to operate as an integrated network. What I would consider doing (tbh we should probably do this anyway) is make the HS2 tickets to/from Leeds (say) valid to/from any West Yorkshire station, and similar for all other HS2 stations.
I concur with integrating HS2 with the rest of the country, which makes it much more accessible to the whole country in general.
Surely all that needs to be done is to offer a 'Plus HS2' fare that is more expensive than the classic network fare, but with mandatory reservations perhaps. Such reservations need to be easy to make though, from what I've read on ash39's trip reports in 2020 it wasn't hard work to make last minute reservations on LNER, I think it could easily be done in an app? I've not looked so I don't know.
The important thing there would be that a passenger can do so easily on their own device, and not have to faff around at a ticket office where one exists. My personal view is that if I've got my e-ticket on my phone, I want to have my reservation easily organised on that. Given the multitude of TOC apps available, I don't know how easy it would be to implement a reservation system, but it sure needs to be user-friendly.
Integrated fares, vital in my view. I'm very excited about HS2, the opportunity to race up and down the country guarantees my using it whenever practical. I would even divert to Birmingham on a trip to Manchester just to use HS2, it's a longer trip than using TfW via Shrewsbury but it would be more fun
However, what would ruin my use of it would be if I have to buy my tickets to Birmingham then a separate fare for HS2. The Manchester example I'd expect to do that, but for a Hereford to London (or hopefully one day to Leeds/York/Newcastle) I'd expect a through ticket with a HS2 option.
Anyway! Back to this Chiltern discussion, and I suspect there will always be a market for decent speed Birmingham services. Without a Route High Wycombe fare, I doubt it would have been as popular as it has become though. Said routing will remain popular even after HS2, not everyone will find the new route as useful/interesting/otherwise desireable.
Personally, I get very bored on the Chiltern route, so I'm not a huge fan. Indeed, if it hadn't been for the 67s and 68s over the years I probably wouldn't have gone that way much at all. I do find the WCML South route to get a bit tedious sometimes, if I'm on a slower service, but I much prefer it to the Chiltern line. Even if I'm on a 350 on a low fare!
Will fast Birmingham services remain in the future? I would imagine so, although I have to say I do miss the days of it being non-stop Bicester North to London Marylebone. I don't miss the slow turnout into the old platform at Bicester North mind, I'm glad that was fixed!
As for the Chiltern route to Oxford, I've never done it throughout however I have seen how incredibly busy it used to be so I'd certainly keep that going. How busy it will be when non-essential retail returns remains to be seen, but based on both my own feelings and that of just about everyone I work with, I will be shocked if it's not back to normal very quickly. I'm talking Day 1, Morning 1 of the end of lockdown quickly. I also don't see such a level of demand dropping too fast either, indeed I predict the railway is going to be very busy again very quickly. Not so much in the traditional peaks, perhaps, but those of us not on a Monday-Friday 9-5 style job will certainly be there in no time! That's a different topic though...
In summary then, my view is that there will inevitably be a few changes to Chiltern's services long-term but even after HS2 launches I suspect it will be mostly in the traditional London commuter areas. Their longer distance stuff will probably continue much as currently