Counterpoint, Meta is currently (and for the last decade) trying to rewrite MySQL so it is basically Postgres. They could just change their code so it works with Postgres and retrain their ops on Postgres. But for some reason they think its easier to just rewrite MySQL. Now, that is almost certainly more about office politics than technical matters...but it could also be the case that they have so much code that only works with MySQL that it is true (seriously doubtful).

You are just mislabling good architecture as 'premature optimization'. So I will give you another platitude... "There is nothing so permanent as a temporary software solution"