It seems like a fair valuation to me. I can see a path for them to approach or surpass Uber's revenue (~$50B) in the future, and I think their technology and brand are actual moats in comparison to all other driverless systems out there.
If Uber goes away, Lyft or others can take over the entire market overnight, precisely because they don't have their own fleet or unique technology. Waymo is placing itself as first mover into a completely new category of transportation which will require capital investment and new tech, so it will be much harder to displace once it gets going. It could target automated cargo transport in the future too.
I don't think there's ever been a giant centralized global taxi fleet.