Just a small point of disagreement: Often, in busy parts of NYC, hailing a cab is much more convenient that calling an Uber. Ubers often take awhile to get to you whereas a yellow cab can be hailed within a few seconds.

I am interested in the question you pose though - about the social utility of the medallion system. Perhaps there's some benefit in keeping the number of cabs limited in order to keep the roads emptier and thus more usable?