> If I can use cockroach or yugabyte and get 90% of the benefits of postgres without significant additional cost I'm going to use it

That's a big "if." Idk about those two, but you sacrifice a lot using Spanner. And in the initial phases, you can't tell what your actual needs will be at scale, so you'll probably have to rework things anyway. Just using Spanner doesn't solve your scaling problems.