I think the Chilterns can grow partly due to the disruption that HS2 will bring to the WCML and also it will mean improvements as the wcml freight gets sent on the chilterns if the WCML is stuffed or being dug up.It won't. HS2 might be of some very limited advantage to the fortunate few who are able to use it. It will be of no benefit at all to the majority who live in the areas it passes through, who will be unable to access it as there will no intervening stopping points. Any capacity that is freed up on the WCML will not be optimised as all available investment potential will have been used up for the forseeable future on the HS2 project.
Will the slower Chiltern Line route to Birmingham (which serves many intervening areas en route) be viable any more?
The capacity on WCML will be used up really quickly but that isn't the point, you need to hold white periods in your timetables to allow engineering work and maintenance. This is whats driving digital railway design to allow fault finding whilst reducing actual lineside disruption