What happens if you (the contributor) fork the project, make a really great feature in your own fork, make it available as the license requires, then the startup integrates changes from the fork back into their own project. Does this then prevent them from ever changing the license away from AGPL?
Yes. If they do it they will be able to license the form back as AGPL but that wouldn't let them license the new thing under their commercial license (effectively useless commercially). At that point they are a peer with you.
Some startups may not know this or knowingly violate this.
