I think you mean: If anybody anywhere decided to invoke a wait() until the state "my COMMIT; has been durably committed", then that means all previous commits must also be in durably committed state before we can continue.

Sure. But this doesn't affect other code that is merely wait()ing for 'logically committed'. Not in the past, and usefully, not in the future either. The 'logical system' can be far ahead of the 'durable' system, it doesn't have to wait.

Where it would go wrong is if code A needs to wait around for code B to do a thing (such as provide a value), and B's code for some reason contains a wait() on durable. Possibly because B was written before the split in commit behaviour was around, and for backwards compatibility reasons, for them COMMIT; means: "wait for durable commit".

Given that `SET synchronous_commit = off;` works per transaction, I'm kinda inspired here. I can think of a few places where we commit for various logical reasons but I don't need the fsync guarantee.