I hear the argument that migrating from (say), postgres -> postgres + books is easier than moving everything into cockroachdb. But why is postgres + books easier than postgres + cockroach? The latter doesn't require you to write your own database from scratch. (And writing your own database is fun, so if you're doing it for that reason I understand, but thats not what the post says.)
Edit: Ah, I see - my confusion was because I missed the part which mentioned that this is just built on top of spanner. That makes sense!
For a fun fact, CockroachDB was actually started by ex-Squares, so we're definitely very familiar with this exact same argument internally, back in the day when neither CockroachDB nor Spanner existed as a viable option :)