CouchDB 4 may change a few things in this space. It will be based on top of FoundationDB, which should make it a lot better for running "at scale PaaS".
Though from what I'm reading the FoundationDB internals are adding even more compatibility breaks from databases that work just fine in CouchDB 2 and PouchDB.
It's nice to see "we're trying for internals that make it easier to run it at scale", but it's not really a "at scale PaaS" play if there aren't any vendors on the major clouds that aren't IBM's interested in actually running whatever those internals are as a service.
