I don't think there was a suggestion that autonomous cars/vehicles will replace public transit. They are just a different transportation solution to add to the mix and they may be implemented as public or private.
The question is, will they end up being a better solution for enough riders to make investment in a new streetcar circulator less successful financially? That depends on a lot of factors, but in the case of an NSS circulator Mr. Brennan seems to think that investment might not be the best choice. For other transportation problems - say, moving people between Downtown and the Cultural District - the numbers may work out differently.