The cost of building 100 Teslas for transporting the people we'd be able to stick into one bus is significantly higher than what we ever could save by not having a driver - and that's _before_ looking at the extra infrastructure cost we'll have: The 100 Teslas use way more space than a bus or a streetcar (capacity: 200-400), so needs more and wider roads, and will contribute to congestion, instead of easing it.
Self driving trams are already being tested - and are way easier to deal with than self driving cars, especially when you isolate them from the road to some extend. It won't change much in terms of personnel cost - countries which had been reducing conductors in trains ended up replacing them with security, so I'd expect we'll still have paid people in buses and trams, they'll just have a different job description now.
