> What if you actually reduce track / signaling / control / facilities cost massively by virtue of using self-powered autonomous vehicles?

The problem is that it's hard to carry more people into a tunnel than a metro train (excepting forcing everyone to become pedestrians). The proposed replacement for metro trains is so capacity inefficient that you're going to spend more building parallel tunnels, and you'll probably have an even more egregious problem of vertical circulation. Vertical circulation is already an issue of concern on the busiest passenger systems, and trying to move heavy, bulky, low-capacity personal vehicles is far more difficult (requires far more space) than packing pedestrians as current systems do.