As this is a speculative thread: I do wonder whether any future more "joined up" railway could save a path by operating an hourly Paddington-Reading-Oxford-Birmingham. This could provide both one of the 2tph from Paddington to Oxford, and one of the 2tph from Reading to Birmingham. Operated with 8-10 coaches, of course.
Of course, way back in the day this was the pattern. The Reading-Birmingham service in the early 80s consisted primarily of a mix of Pooles and Paddingtons with a small number of Brightons and Portsmouths.
There's nothing fundamentally wrong with XC as a concept - it seemed to work fine in late BR and even early Virgin days - but there simply isn't enough stock.