Why? There's lots of features that Postgres doesn't have, but could, which would fit perfectly well into the current offering. Right off the top of my head, built-in bitemporal support recently added to MariaDB would be welcome in Postgres, and would have no negative impact on any existing functionality. "Stop adding features" is sensible when adding features degrades existing features or the experience thereof, it makes no sense at all when it has no negative consequence.

Some people need or benefit from things that don't interest you. If that's the case with a RDBMS, why not just... not upgrade?