> You need some combination of high-ridership and low-personnel-cost.

Driverless (GoA3) and completely unattended (GoA4) rail operation is possible, which could help with personnel costs:

* https://en.wikipedia.org/wiki/Automatic_train_operation

People conservative enough to drop the required nuclear bomb on the transit workers union to be able to deploy something like this won't pay for it, and vice versea.