If a court rules that the OGL 1.0a is not revocable, then you don't have to trust Wizards. They don't get endless bites at the apple. Frivolous litigation is a thing. (I think frivolous litigation is what would apply in the case that Wizards kept hoking up increasingly outlandish excuses to challenge the OGL in court, but IANAL.)Perhaps, you also have to factor in whether anybody trusts WOTC to abide by an agreement they made into perpetuity. Because nobody wants to be here again in another 20 years. That's the bigger factor, I think.
This is why I suspect Wizards will think long and hard before filing any lawsuit that could result in such a ruling. The threat that they could sue serves them almost as well as if they'd filed suit and won, without the risk that they might lose, or the storm of bad publicity that would hit them either way. Per our resident lawyers, there seems no plausible way for a 3PP to take the initiative here -- Wizards merely announcing "We have deauthorized 1.0a" doesn't give anyone else a cause of action to haul them into court. So Wizards can hold that threat suspended over everyone's head as long as they want to. Maybe toss out a C&D here and there to keep anyone from getting complacent.
Of course, if there is a big 3PP that is publicly and loudly defying Wizards, that forces their hand. If they don't file suit, they are revealed as a paper tiger and the threat loses force. But most of the big 3PPs are extricating themselves from the OGL. Wizards can deal with the little fish at its leisure or not at all.