Wow! I loved my Contracts class. And I think our disagreement highlights the difficulty of the question.
The reliance principle might be enough to overcome parol evidence issues, but there are other issues which, in my opinion, tip the balance away from irrevocability. 1. In licensing law, courts have expressed a strong inference against irrevocability unless irrevocability is explicitly stated. 2. The consideration given by licensees to WotC is exceptionally weak. They only have to agree to abide by the terms of the license, which basically reiterates what they are already not allowed to do under copyright and trademark law. In return, they've been given access to WotC material that has given some of them a living. (In in WotC's best interest to foster a healthy fan community, but that is beside the point legally speaking). Consideration makes a contract binding, no matter how minor, but as a factor in determining the strength of a reliance interest, it does not help.
If I'm not mistaken, trademark law would allow us to indicate compatibility, etc., something explicitly restricted in the OGL. That is, I can't say "compatible with D&D 3e" unless I have a separate license with WotC saying so. This is a consideration, yes?
Ditto, by using open content I have to allow others to use my open content.
OGL v1.0a is generous, to be sure, but does have considerations both ways. If it didn't, it couldn't be a contract or license, right? They get something, we get something. This is, if I'm not mistaken, a big part of the reason for token payments such as a $1 retainer to a lawyer, to establish client-lawyer privilege... a contract doesn't need to be objectively even, just acceptable consideration on both sides.
And as I have mentioned above, this being an open license essentially makes it a unilateral contract, which has its own weird rules about offer, acceptance, and revocability.
Third, looking at WotC's language in the Q&A, they are not, strictly speaking, claiming the license is irrevocable. They could say they were merely describing the status of the current license (which does not revoke prior licenses), which does not preclude a future license that does revoke prior licenses.
How does Section 9 measure up then, that says "if we update the license, you can continue to use whichever one you want"?
It seems to mean that (for example) if v1.1 says "v1.0a is no longer valid", but I do not accept v1.1 (and according to the license I have accepted, I don't have to) then v1.1 does not apply to me.
If they took it to a different meta level and revoked 1.0a 'publicly' then it seems clear I'm out of luck, but if it's done via a license I don't accept, does it still apply?
Hmm. Even then I wonder, if they stopped using v1.0a themselves even for stuff they licensed that way 20 years ago, but the license is still valid for those works and lets us sublicense, what does it mean for those cases?
(as you might have guessed, I'm not worried about 5e, I'm worried about stuff I've been working with for longer than 5e has been a thing...)