GPL wouldn't made a difference here, since it's a web service. The AGPL story is different. I think you are too trusting when you say that the creator goodwill will prevail - as an author, you are better off making this enforceable, even if you don't plan to sue anyone. Also, one can go the MySQL way - dual licensing. You end up with more options in hands by using AGPL, and that's a good thing.