That's not an accurate description of the argument.
It's not in dispute that WotC can revoke their licence to offer at any time.
The question is how - if at all - this affects the interpretation of the phrase the Open Game Content as that occurs in section 4 of the OGL v 1.0/1.0a. The phrase needs interpretation. Its interpretation is in my view not obvious, because of the interplay between it, the same phrase in section 3, and the phrase any Open Game Content in section 2. As far as I know, I'm the only person in this discussion to have explored this issue.
So long as WotC's offer remains on foot the question is of only academic interest, because any licensee can always go back to the SRD and draw on more of its OGC that they haven't yet used. But as soon as the offer is withdrawn - which, to reiterate, no one disputes is within WotC's legal power - then the interpretation matters. Because until the interpretive question is settled, we don't know what the subject matter of the grant in section 4 is, and hence what power if any licensees retain to sub-licence in the event of a withdrawal by WotC of its offer.
As to your point about risk, I posted something like it - I guess with more of an abstract legal focus than a commercial one - about a decade ago on these boards. So I agree. At that time I didn't try to work through the interpretive issue described above - I don't know if I even thought of the issue, and in the intervening 10 years I have worked pretty hard on theories of interpretation (particularly involving words like "any" and "the" as they appear in this sort of instrument - these give rise to interesting interpretive complexities) and so can probably now think of things I couldn't have worked through so clearly back then.
As to your first question: if the offer is made pursuant to contract that makes a difference. Hence why I think the 3E-era SRD may be in a different situation, at least vis-a-vis WW and Green Ronin (I believe those are the right parties), as WotC used their OGC in some of its works. (It's no surprise to me that WotC stopped doing this. It seems commercially foolish to do this sort of thing and hence become just another member of the network rather than a source from which all licensing flows in one direction (ie outwards).)
You could also draft in such a way that the power of sub-licensing isn't hostage to a withdrawal of the licence. I'm not a drafter - my expertise is in interpretation - and so I haven't tried to work out what that might look like. But a starting point would be eliminating the possibility that the licenced content (ie the OGC) consists of content still subject to an offer to license it (ie, on one reading, any OGC that contains a notice indicating that the Open Game Content may only be Used under and in terms of this License). I mean, even changing that verb "contains" to "contained at the time of licensing" might be enough.
It seems fairly clear to me that, when the OGL was drafted, promulgated, and taken up by 3PPs, no one on either side of the agreement contemplated in any practical sense WotC withdrawing its offer to license. Even the stuff on the FAQ about revocation I think is more focused on WotC trying to revoke issued licences rather than WotC withdrawing its standing offer. (At least that's my recollection. I'm happy to be corrected if my recollection is false - I haven't gone back to the FAQ text to double check.)
