The process that would stop 100% in-use cabs from being diluted into fives times as many 20% in-use cabs is that the 20% in-use cabs would all be losing money.
If congestion is the problem, then directly charge for that - London has for example implemented a time-based congestion charge for the city centre.
If congestion is the problem, then directly charge for that - London has for example implemented a time-based congestion charge for the city centre.