I would say regardless of exactly where the bottleneck is for the sake of his conversation you absolutely will see some paid relays use client auth when it is widely available. Otherwise your large “read” costs will always be heavily subsidized by a small pool of writers. Long term those economics won’t work for most paid relay operators.

This is a problem that will have to be dealt with but I don’t know that it’s unreasonable for the short term solution to just be you may not (necessarily) receive 9735s on those relays.