One solution that wasn't mentioned in "Possible improvements" is "provide a disk format that doesn't require pg_upgrade on every major release". I'm sure it's

One solution that wasn't mentioned in "Possible improvements" is "provide a disk format that doesn't require pg_upgrade on every major release". I'm sure it's not that simple, and there are probably some benefits in the current approach as well, but I'm not so sure if "require pg_upgrade on every major release" is really the best trade-off here. As mentioned, it can be a major PITA.