Although I admit that you could use the arguments you put forth to make a case for regulation of the taxi industry I think you'd have a hard time justifying the existing medallion system based solely on those reasons.

The medallion system (like the car dealership system) is made to protect the incumbents. Period. There was no a priori argument that this was going to be good for the people of the city or that every citizen could expect a predictable rate and level of service. In fact, the complete opposite was true -- these regulations were born out of period with a glut of taxi labor and falling rates.