I note how the stations inside High Wycombe have barely got a mention.
On any other line in the South East, the likes of Northolt Park, Denham etc would have a 2tph minimum service, but obviously Chiltern have gone for the much more profitable Oxford etc. services in preference.
Well inside High Wycombe you have got Beaconsfield with 3tph each way and Gerrards Cross with 3tph north and 4tph south (+ 1 terminator).
The big difference though is capacity - pretty much every other line into / out of London has more than 2 tracks available to it, which allows faster trains to get past stoppers, whereas the Chiltern line doesn't - part of that is BR legacy of removing passing loops, but also it was only ever built as a 2 track railway and never upgraded to 4. And under BR from the late 60s through the 1970s and first half of the 1980s it was probably more neglected than pretty much any other route into / out of London - BR totally failed to see any commercial value to the line - it's easy to forget that at one point places like Banbury received a stopping DMU service every 2 hours from Marylebone - when compared to places a similar distance out from London, such as Peterborough or Kettering - that was a particularly poor service.
I think that Chiltern will keep a significant Birmingham - London service, even post HS2, because it's less about end to end journeys and more about the other places served en route. There are people at places like Leamington, Banbury or Bicester who want to travel to both London and Birmingham - HS2 will do nothing for any of those except maybe make their trains a bit quieter. So any attempts to reduce the service pattern or increase the journey time isn't going to be well received.