A completely reasonable stance.

However, it's not so much a performance question as one of concurrency. The easiest way to get the right semantics is by locking the entire destination table against concurrent writes. Maybe that's fine for a lot of users. But it has caused some hesitation, because other users might be disappointed.

It would be fair to say "just do it" though.

Part of the motivation behind PgREST is to get a reusable and generic enough upsert implementation as a stored procedure; see https://github.com/pgrest/pgrest/blob/master/test/upsert.ls for details.