>"Either the driver rented the car to a middleman who rented the medallion from a rich owner, or said owner was selling and financing (most banks won't touch these medallions!)"

There are plenty of commercial banks that have underwritten loans for taxi medallions and built up a significant portfolio Just in NY for example - North Fork Bank, Capital One, Provident and Signature Bank, See:

https://www.autofinancenews.net/allposts/finance/loss-mitiga...

https://nyrej.com/print/18823

https://www.businesswire.com/news/home/20200117005338/en/Sig...