Being an extension also makes you entirely dependent on somebody else's platform, and makes it possible that your work will simply be subsumed by the platform if they think it's important enough. It's a very weak business position to be in and you have to have incredible future looking planning and brand buy-in to make sure you succeed like this.
You're already dependent. Upstream can turn around tomorrow and provide everything your fork/extension does for free. They can alter their entire codebase to cause you weeks of work to keep up. It can be a hard slog being downstream, no doubt about it. That's why downstreams tend to "get involved" upstream. Sponsorship, sit on technical advisory boards, etc.
But what you're saying —which wasn't immediately obvious, and correct me if I'm wrong— is your users are using your database product, not Postgres, so you can hold them back as long as you like when they're using a forked product. They won't be carried away by an automatic update and it's much harder for them to jump ship.
And while there is some truth to that, it comes with a karmic cost. People picked you because you were based on their favourite, industry tested database. If you slip behind in features, or (more importantly) can't backport security fixes instantly, you're dead.
