That's the big thing - PSQL will be tested, noticed, and fixed (and likely have a version that handles 7.0 by the time it's in common use).

But other software won't and may not even be noticed, except as a (I hate using the term) enshittification.

Better to introduce the "correct way" in 7.0 but not regress the old (translate the "correct" into the old if necessary) - and then in 8.0 or some future release implement the regression.

Exactly, this is how it’s usually done. As the developer on the mailing list mentions, implementing a new low level construct in 7.0 and a performance regression that requires said new low level construct to mitigate is not great. You need a grace period in which both old and new approach is fast.