Gratiutious promises generally aren't binding in common law legal systems. That clause only binds WotC if it becomes a party to a contract, which only occurs when someone takes up the offer.
Yes, but you see, since the license is, by its own terms, distributed with the licensed work, WotC cannot recall it. Remember that entring into this license agreement is
passive on WotC's part. There is no active communication between licensor and licensee, and thus no way for WotC to change it.
Ryan Dancey was not incompetent. But there is no simple way in common law legal systems to unilaterally make a standing offer to all the world that is not capable of subsequent retraction.
Oh, yes you can, and in fact, must be able to, or else the following is possible.
Say Joe Writer creates the "Open Author's License". Its terms are equivalent to putting the work into Public Domain, with the one stipulation that the work must include a frontispiece reading, "Many thanks to Joe Writer for his material," with the work. Joe publishes a book, including v1.0 of this license.
Then, the week after Joe creates version 1.1 of the Open Author's License, with the same stipulations, but an addition that the licensee must also pay $100,000 per month to Joe Writer or his estate, in perpetuity (or, include any other onerous stipulation that you wish - for the example, all it needs is to be onerous). Joe publishes a single copy of the book with this new license.
Sam Author comes across the work with version 1.0, and uses it. Joe comes up and now claims that he's retracted 1.0, and that Sam now owes several hundred thousand dollars worth of pack payments, or other imposition of the onerous terms, or face lawsuit.... Which is, of course, complete nonsense. You cannot change contract terms without the permission of the parties in the contract!
So, no - if you openly distribute a work with a license, and allow the licensee to enter into that license without an express discussion and signed agreement with you first, the license can't be changed in the background, separate from the work it applies to. The OGL happens to make this expressly clear, but it would be true regardless.
WotC is free to re-release the same material under a different license. But, that new license is merely another option that
may be used, not
must be used. They cannot prevent anyone from using the older one, because WotC cannot control whether a licensee got a version of the text with the new or old license.
And, in fact, WotC did this - back in the day, 3e was licensed under the OGL, or the d20 license, depending on whether the publisher wanted ot use the d20 logo and actively claim compatibility with D&D or not.