Yes, I'm aware of the parol evidence rule. I teach it.
It means that if the written contract document is clear, then pre-contractual negotiations won't affect the interpretation of what is on the page. But here we have a different situation (IMO). One where WoTC and 3PPs spent over 20 years acting on the basis that the document meant X, everyone thought it meant X, and now the new WoTC people say it really meant Y.
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.
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.
It would certainly be strong evidence against them in court. It may even tip the scales towards irrevocability. But I just don't see a United States judge saying that the 5e SRD must be open for all time.
Gosh, this is some good debate.