True, that would be an extra bonus if Heroku started allowing replication to non Heroku instances, but as long as they support logical replication to a Heroku Follower instance then you can upgrade to new major versions with near zero downtime - set up logical follower running latest postgres version and then promote to master once it has caught up. Currently you can't have a follower that is a different version to master - meaning an upgrade requires either a full backup and restore to new version resulting in significant downtime if you have a large database or using the pg_upgrade utility which is generally not recommended as it is not guaranteed to work.