[martinced]

"How difficult would it be to switch out Datomic for PostgreSQL?"

Seen that Datomic can be backed by many different types of storage, including SQL DBs, this sounds like a very weird question... I mean: if you want SQL, just use SQL as the backing store for Datomic.

The benefits of Datomic are too huge to go to the old crappy way of designing DBs...

[ryanbrush]

The desire for an open source app top-to-bottom or the need to use existing databases or tooling would be a good reason.

(However, I do think this question is a bit of a red herring: I don't see any obstacles to pulling in a different database in Pedestal.)