Environmentalists should be happy about this either way. A fleet of high utilization autonomous vehicles will increase utilization rates of each automobile that is still on the road substantially, serving more people with fewer raw materials. Not to mention that as of right now, all of the leading contenders for commercially viable robotaxi fleets are on EV platforms anyway.
It's not that, by and large, over a longer time horizon, new gasoline cars are going to replace these EVs disappearing from the consumer-owned automobile segment so much as EV robotaxis will be gradually replacing almost all consumer-owned vehicles. Enthusiasts will still have their track toys, but as an economic mode of transportation, the personally owned automobile is going the way of the horse and buggy.