Why is it ridiculous? If you look at Transport for London as an example, the organization employs 25,000 people, of which 3,000 are train operators, with 600+ trains. The salary + indirect wage costs for those people is on the order of $250 million per year. Which I think is pretty cheap, compared to what you'd guesstimate for CAPEX and OPEX for an automated system.

Now they are actually in the process of automating the trains, which has been criticised by many as a vanity project. Certainly it has not been shown that it makes financial sense. And it has been decided that they will retain a "captain" aboard the train, for safety reasons, as someone has to lead passenger evacuation in case of an emergency. So the total salary savings will be essentially zero.

Fully level-4 automated may makes sense for new builds. But for retrofits, you can't satisfy the overall requirements without a human physically present.