Thanks for sharing your point of view. I just find that there's a lot of confusion in this article so I wanted to give my reaction.
In particular when talking about consistency, an open source project doesn't automatically get replaced by forks. The original project would always remain as it is. That's the end user who decides which is the best in practice. Rejecting PRs will always be possible if you care about it, just as you're already doing.
Plus speaking of licence contamination whether you use an MIT-like licence or any non-open-source, if there's some "copyleft" material being used you must comply. That has nothing to do with the licence of GreenSock project.
And I don't want you feel bad for not making it open-source, that's what we call usually call freedom anyways.