I feel it's worth reminding that there's a difference between autonomous trains, driverless trains and unattended trains. The L train is considered Grade of Automation 2 - it starts and stops itself, though it has someone in the driver's seat who controls the doors and gives control to the ATO system. Driverless (GoA3) trains move that role to a train attendant who controls the doors from within the train. Unattended (GoA4) trains function without needing an on-board operator at all.

The driver probably is just sitting with a hand near the emergency stop button most of the time, but they're still required to pay attention. I imagine it's harder work now that they don't have to necessarily concentrate to operate the train.

I live in London, where a lot of tube lines are already (or soon to be) GoA2, with the introduction of a part-GoA2 mainline service this year (Thameslink). I don't think we'll see a GoA 4 tube for many years; aside from the obvious union rows, you've got safety concerns (primarily platform-edge-doors, which can't be retrofitted to cramped stations), and there's mixed public opinion on the idea of being stuck on a broken down train in a tunnel without on-train staff. Then again, people don't seem to acknowledge that the airport terminal transfers are unattended GoA4.