How often does one change the DB backing a live production application?
In one of the cases where I had to switch, we swapped from Cassandra to S3 for 100x OpEx savings since C* couldn't scale cost effectively to our needs, so we rolled a database on top of S3 instead that well out performed C* for our use case (e.g. need to export a 3B row CSV in a minute?).
If its easy to do this then you are using a tiny fraction of Postgres.
If you want it to be easy to switch your database then you need to code to the lowest common denominator. I would rather use my databases to their fullest potential, rather than purposefully handicap myself because I might have to change it in the future.
I used to design systems so this was possible, but eventually realised it just wasn't needed - I was adding more abstraction and complexity for no reason.
I took it as changing the DBMS under the hood.