Well the reason light rail and subways are so good isn't because they're on rails, but because they get their own track. If you put a bus on its own street, suddenly they're not as bad anymore: https://en.wikipedia.org/wiki/Bus_rapid_transit
They're also good because of the commitment involved. You can build around a light rail or a subway stop, safe in the knowledge that it will still be there in ten or twenty years. You can't build around a bus route like that. Ironically, the high expense and lack of flexibility are features, not problems.
I agree that BRT (which I have a lot of experience with)[1][2][3] does make things a bit better. However, the successful BRT I have seen is always in very specialized circumstances, while the proposals I have seen for BRT in (for instance) San Francisco have been so full of weird compromises and bizarre edge cases that I am not optimistic they will make anything better.
