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.

There are already driverless systems deployed in Canada and Europe where the transit service is probably unionized:

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

Existing drivers could be moved to customer service roles on the trains (driver+conductor on 1 train -> one coductor on 2 trains), or moved to non-autoamtable roles (buses, trams).