I don't mind giving up the road to cyclists in cities. I just hope that one day they are finally treated like cars. Bikes on city roads should have to be registered with plates. Bikes in many places in Japan are registered and it makes it a lot easier to figure out when someone is riding a bike they aren't supposed to have. The plates would hopefully go along with police actually enforcing the traffic laws on cyclist…
But let's go ahead and agree with you and add number plates to bicycles - ignoring the huge costs incurred by the state - how exactly does that help enforce any laws? Bicycles are to small and light to trigger most red light cameras and to slow for speed cameras. So you're only left with visual enforcement by an officer of the law, which will have to physically stop the offender to issue any fines... which... o, hey... you don't need a number plate for that.