Coming from the outside, with zero understanding of the internal details, my hunch is the same: lack of support for logical replication is more of a business decision than a technical decision. (But again, this a hunch -- partially based on how good heroku is from a technical perspective)
It's absolutely an evil business decision, and all the clouds are playing this game. Don't ever use a hosted database solution if you're thinking about storing any significant amount of data. You will not be able to get it out without downtime.
