Yes. The difficult problem is "antisocial behaviour". Also, to be fair, we might call them people who happen to be experiencing mental health incidents.

What are you going to rob? There's no money.

Most PT systems recover hardly anything from fares, the fares are a proxy for "are you a functional person". Would you return a shopping cart?

You can't make the bus or tram (at scale) fully autonomous for safety reasons. Societal trust isn't high enough. When you have a situation where there is a vulnerable lone woman or child victimised the situation will be politically unacceptable. Maybe AI surveillance and targeted enforcement can paper over this.

If you need guard labour though I suppose they can be more effective if they don't have to drive.