The problem with that is that there is a supply of cab drivers and demand for cabs that drives the price. If you put in a medallion system, the price of the medallion will be bid up such that the cab driver's wage is in line with the market wage. You can't just wave a magic wand and set prices without unintended consequences. So what happened was cab drivers had to take out massive debt to finance these medallions or work for some middle man that is essentially a financing arm. And when the price collapsed, they were stuck with this debt and some even got bailed out by taxpayers.

I wish Uber was around when I was younger. I had a car and a lot of spare time. I would have gladly accepted a low wage if I had a few hours to kill. No other job affords that flexibility which is probably why its so popular.