the public bathroom should be redesigned so that one can do only the thing it is designed for - and maybe it needs to be a single-user design.

Almost like an airline bathroom size - might be a bit claustrophobic but these are for emergencies and space saving would be big.

the doors could be wired to sensors that are connected to a central network that shows in real-time the availability and also some alarms if something is out of the ordinary etc.

Yeah you have to actually hire people to clean them and patrol for homeless weirdos. Which is why in my country they cost money.

I never really understood the economics of public restrooms in America. Who pays for them?