I observe that 1.0 contains the verbiage:
"Updating the License: Wizards or its designated Agents may publish updated versions of this License. You may use any authorized version of this License to copy, modify and Distribute any Open Game Content originally Distributed under any version of this License."
which at least to my open source license not-a-lawyer read says that they basically can't revoke it.
They can relicense stuff going forward under 1.1, and that may be bad, but I wonder if people are misunderstanding and treating it as apocolyptic when it's just bad. It seems like the worst case scenario is the community undergoes a de facto fork and you can stick to a 1.0 world if you want, not that all the 1.0 stuff goes away.
This smells to me like many similar panics in our community when some project goes to relicense and people don't generally understand that the relicense only applies going forward, because in general you can't retroactively relicense an open license. (There are nuances to that statement which I'm skipping over, but I'd say that's the most correct short summary. The thing that people think is happening is not what is happening.) Of course it is still valid to be upset about the relicensing going forward! It just may not be quite as much a kick in the teeth as people think.
Apparently they are. There is some uncertainty about their right to do so. The few lawyers opinions I have read tend to say they probably don’t but it’s not entirely clear cut. Someone would have to go to court for it to be ascertained but I doubt anyone wants to fight Hasbro for some old content generating no revenue.
Its complicated.
The general law of gratuitous licenses (in the US) is that they are revocable at will; terms of the license do not overcome this. Whether the restrictions in a license like this are limitations on the permissions it grants (leaving it gratuitous) or consideration in exchange for permissions (making it a license contract) is not always an easy question.
Further, even if things for which the license has already been uses might be protected, the license text is an offer, and the offer can be withdrawn. This is important, because the OGL 1.0a isn’t sublicensable; every new user making a new work directly licenses all ancestor works under it, rather than sublicensing through the immediate parent.
But, even to the extent it is recovable, the doctrine of promissory estoppel may limit the effect on parties who have already reasonably relied on the promise of irrevocability.
The OGL does explicitly use the phrase "in consideration for", which you'd hope would make this non-gratuitous. Then again, IANAL.