Again, with your hypotheticals—when was the last time you needed to do any of that with Postgres or another FOSS DBMS?

For the vast majority of use cases, a FOSS DBMS and a free-as-in-beer DBMS are indistinguishable. If you're in a category where they're not, then don't use Datomic, but this is still far more than a publicity stunt.