Under that logic, if a publisher of original content thinks a reuser is violating the OGL, it would have to convince Hasbro to take action instead of being able to take action on its own. That's not how open licenses work. Lawsuits filed over violations of the GPL are between the originator and reuser. The Free Software Foundation is not required to get involved.
The terms of the OGL v1.0a are unambiguous enough that I question whether there'd be an issue of anyone "thinking" that a reuser is violating the terms; they either would be or wouldn't be (though to be fair, people can certainly find new ways to creatively interpret almost anything).
That said, the problem with the "reuser is the one who enforces action against a violation" theory is that much of the time, violations aren't about people whose content has been reused at all. In fact, you can violate the terms of the OGL without there even being a reuser. For instance, if someone publishes an entirely original RPG, using no Open Game Content from any other source, and releases it under the OGL but fails to include a notice outlining what parts of their work are Open Game Content (which they have to do as per Section 8), then they're in violation of the OGL. Who enforces that provision, then? The answer can only be Hasbro.
Now, it's possible that we're both right, and that a reuser could cite a violation that has to do with their own Open Game Content, Product Identity, trademark compatibility notice, etc., but I can't see an instance where Hasbro wouldn't have standing to do the same, since it all happens under
their license.