Anyone who has taken up the offer and entered into the contract has rights which are, according to the terms of the licence, irrevocable. Whether those terms actually govern the agreement or whether there are other legal rules affect the term of the licence is something I don't know. @S'mon?
As far as Frylock is concerned, he has long been a critic of the OGL on the grounds that, under it, WotC asserts IP rights that it does not actually enjoy and gets people to contract away rights that they otherwise would enjoy. So I don't think he is interested in himself becoming a party to it.
The general approach to a contract under US & UK (English) law is that the court will seek to uphold the agreement. Partly under EU influence UK courts will regulate business-consumer contracts made under standard forms from the business, but the OGL is not such a contract IMO.
Frylock is technically correct that WoTC claim rights they don't have - eg we all know the Displacer Beast is really a Couerl from AE Van Voght - but the OGL does also grant substantial rights to the licensee.