Without major cultural shifts, you still need someone on the bus to prevent them from becoming moving dumpsters. The driving itself would be much easier, as you point out. That's not the only thing that a driver does.
That would be much cheaper than a union bus driver who can clear 100k.
Safer too since bus drivers often go tons of overtime which isn't great for being alert.
https://news.ycombinator.com/item?id=30856522 (citations)
That's typically enough to stop most weird behavior like throwing trash on the ground.
This isn't exactly a direct source, but it's listing the hourly pay at 25.26$.
You wouldn't even need them to ride every single bus, just enough to deter weird behavior.
Even if hypothetically they cost as much as a real bus driver, this would be much safer. A bus driver doing overtime and possibly texting people isn't too hard to beat.