Checksums were introduced in 9.3, yet not made the default until 9.6. Converting it requires lengthy downtime, back to the original author's point of major version upgrade pain.
Everything worthwhile in PG is introduced over a long time, requires a lot of pain to adopt, and if not adopted quickly, suddenly becomes "lol why aren't you doing this it's been there for years." That's the disconnect between people who actually run these clusters and the somewhat ivory-tower views of the postgresql devlopers.
Maybe you should learn and understand why people are happy with postgres in the first place, and then understand your complaints in that broader picture. Then you wouldn't resort to mockery and other unproductive comments.
I don't understand why it takes 4 releases before a tool like pg_checksum becomes available after the larger feature is introduced. If the whole thing isn't ready, don't release it.
I'm not saying it's easy. I never said it was.
It is also available as a Debian package for the above versions via apt.postgresql.org.
[1] https://www.postgresql.org/docs/current/app-pgchecksums.html