The solution might be to move to separate Postgres boxes everything that needs performance tuning at all.

I'm not in the "use Postgres for everything" camp, but only because I think it's too complex to be used like that. It should be replaced with a bunch of simple primitives in this role. No SQL and query planning magic please.

YES!

The problem of all the datastores is that are applications (like Wordpress) so you are too late to fix anything deep.

We need "frameworks" (so each sub-component can be used as-is or even swapped) and even wondering what a "system level" data engine could be.