I don't spend much time thinking about transit but I believe the dynamics of it will change dramatically when ever car is self-driving and whether western city governments finally allow the sort of building density and pace that is in demand.

I don't see much difference between a more affordable autonomous bus or van option vs a subway. The benefit will eventually be access to the tunnels more than the fact it's large and on a track. But in cities it's also incredibly expensive to build anything without a hundred stakeholders interfering so a large amount of new tunnels is probably DOA just like most high speed rail projects have been.