I am a lawyer. I have no professional expertise on this topic, have not done any specific research, and am not providing legal advice. This is based on my understanding of general U.S. contract law and other lawyers I have seen/read discussing this issue.
As an overriding principle, where a contract's terms are clear, a court will not look beyond the four corners of the document. However, if terms are missing or ambiguous, a court will try to determine the objective intent of the parties as to that term - i.e. what the parties would would have said if the issue had been spelled out in the contract.
Here the OGL 1.0 is silent as to whether or not it is revocable. (Alternatively it is ambiguous as to whether "perpetual" revers to revocation or merely duration; I think the overall analysis is the same either way). Thus a court would look at the intent of the parties on the issue.
Without delving deeply into the facts, I think it is clear that the licensee and licensors believed that OGL 1.0 was irrevocable. The FAQ and statements by Ryan Dancy discussed in other threads are clear on this. Other considerations a court could look to on this are the course of performance (WotC did not revoke the OGL over twenty years, despite revising the terms for 4E, and even when Pathfinder eclipsed D&D in the market; this suggests that it was understood to be irrevocable) and the reasonable commercial expectations of the parties (no reasonable company would build their entire business around licensed content if they understood that their license could be revoked by their main competitor at a moment's notice).
Yes, there are cases holding that "perpetual" licenses can be revoked at will. This seems to be the basis for other lawyers who have expressed the opinion that OGL 1.0 is revocable. However, without having reviewed those cases, I would assume that those courts did so based on an analysis of the specific facts at issue there. Put differently, based on my understanding of the principles of U.S. contract law, I would be VERY surprised if any case has held that were a contract is silent as to revocability, it is per se revocable, even where the parties had a contrary intent.
In sum, I believe OGL is irrevocable; while the actual license is silent, the parties' intent that it be irrevocable is clear. That said, WotC will be able to cite a number of cases holding (on different facts) perpetual license agreements are revocable, so they should be able to present at least a colorable case if they want to challenge this. I wouldn't underestimate the ability of savvy lawyers, in a high-stakes case such as this from putting up a good fight.
As a practical matter, I think any litigation between WotC and the major players (e.g., Paizo) will settle. In my experience most companies are risk-adverse; where stakes are very high for both sides, and neither has a clear slam-dunk case, they are going to settle for a compromise they can live with rather than rolling the dice. This means the revocability of OGL 1.0 may remain an unresolved issue that deters publishers from using it for a long time, even if the argument for it is weak.