"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...
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"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...
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.)