Subprime is less than 20% of car loans, because it's easy to get a car loan if you have good (decent, not great) credit. Car loans are relatively safe because they are collateralized. This is why my bank is offering new and used car loans at 1.99%, and manufacturers/dealers can offer even lower rates. Even with poor credit you can get loans for less than 10%, which is fantastic by historical standards.

Even for the drivers who have terrible credit they can get still get a subprime loan and keep driving. Even at absolute worst case 20%, that's $6,000 a year, about $3/hour for a $30k car being used full time for Lift/Uber. Leasing a cab from a medallion holder for one shift a day probably cost close to $10,000 a year. In the case of the sub-prime loan you can refinance when your credit improves or pay it off over time. The Medallion lease costs were forever. And the driver likely can get higher utilization with Uber and Lyft than they ever got with their cabs, so they should be able to make more money.