Like many efforts (effective or otherwise) to solve environmental problems, fixing this might require new tyres sold to environmentally conscious consumers initially, to prove that it can be done. Cost effectiveness comes later, paired with government regulations.

I'm assuming there are currently no tyres on the market that contain nothing that degrades into microplastics, but please correct me if I'm wrong.

Streetcars, subways and LRTs do not require new fancy tire compounds to stop releasing microplastics. Their brake pads can, though, although I imagine that regenerative braking helps somewhat.

Bicycles are so lightweight that the amount they release is negligible compared to a car.