In some ways I agree, but that would seem to suggest a no growth scenario and even if domestic travel declines, I can see international, particularly long-haul increasing in future years. That would fill up any capacity freed from domestic services.
Also, I can't see any more than a London-Birmingham-Manchester/Liverpool high speed link being built in the next 15 years, so we won't see all domestic capacity moving on to the rails.
It has always struck me as particularly dumb that when the Heathrow Express rail link was built they didn't allow for trains that could go to Reading and on to Bristol/Cardiff or Oxford/Birmingham.