I think this is the key question that would need to be resolved by a specific court in a specific case.
No doubt past statements by former WotC employees would play a role in resolving the question!
It's not really possible to predict the course that future litigation might take.
But if I was setting this is as an exam problem, here's roughly how I would see some of the key issues breaking down:
Identify offer and acceptance: an offer to the all the world (a la Carlill v Carbolic Smoke Ball Co), with acceptance established by the appropriate mental state accompanying use of the SRD content. The original licensor can retract that freely-made offer at any time.
Identify consideration flowing in both directions: the licensor promises grants a licence to use their OGC (as defined by the terms of their offer ie the licence text); the definitions of use etc mean that this includes a power to sub-licence, which seems to be reinforced by the reference to sub-licences in section 13. The licensee, in return, promises to offer to the world to licence their OGC on the same terms (and unlike WotC as original licensor, they are bound not to retract their offer), and also promises to refrain from exercising certain privileges of use they might enjoy in respect of some of the licensor's trademarks and/or copyrights (to spell this out fully would mean going into the definition of product identity, and into doctrines around fair use and use of a trademark without passing off, etc).
This discussion would carry more marks than the discussion of offer and acceptance, but ultimately doesn't seem that difficult.
Identify other interesting legal features of the contractual regime created: one of these is the power that WotC enjoys, under section 9, to promulgate variant licences, and the concomitant power of licensees to choose which licence to use when they use others' OGC. There are at least two aspects of this a good answer might tease out in more detail: (i) to what extent must a variant licence replicate the terms of the OGL itself in order to count as a "version" for section 9 purposes (a good answer would approach this in terms of (i)what contractual permissions has a downstream licensor granted to (a) WotC to establish new terms for downstream licences and (b) downstream licensees in respect of their use of the licensor's OGC); (ii) what if any legal consequences flow from the different wording used in section 9 compared to section 4 to describe the licensee's entitlements (ie is this of legal significance, or just a drafting infelicity)? I think this is harder than either of the above points - the text of the contract doesn't take it all that far in my view, and more thought has to be given to establishing a plausible account of the contractual regime the parties are creating. Concepts of "reasonableness" might have some work to do in this analysis.
Another of these interesting features is the way that section 13 operates to preserve sub-licences even in the event of termination for breach. I think this is easier than the section 9 analysis, but there is scope both for technical work (ie even terminated licensees remain parties to the contract to the extent that their promise to downstream licensees conferring permissions and powers on those licensees remains on foot) and practical analysis (ie the role of this in supporting the network of interlocking contracts that creates the OGC "ecology").
A third interesting question, and the one raised by the OP, is what powers - grounded in their intellectual property rights - do contractual parties retain to revoke or vary the terms of the licences they have granted? Do these follow the contract, or do they obtain independently of the contract but potentially give rise to breaches of the contract if exercised? Answering this requires knowledge both of IP law in general, including the power that parties enjoy under that law to vary the incidences of their own ownership of intellectual property; and also articulating an interpretation of the contract insofar as it bears on these matters. This second part of answering the question would draw on the use of the word "perpetual", the presence of section 13, the absence of an express power of revocation, etc. The past conduct of WotC, both what it said to (actual and prospective) licensees and how it acted or didn't act in relation to them, would be relevant at this point also.
Consider consequences of, and remedies for, breach: if WotC purports to revoke the licence, and on the better view of things does enjoy such a power, but in doing so is in breach of the contract, do licensees get damages for breach? or can they insist that they still enjoy the licensed permissions and powers? Does it make a difference to this if the licensee is plaintiff - and hence seeking a remedy - or defendant to a copyright suit - and hence seeking to simply stand on its contractual rights? Etc.
