Since they grant the right to use OGC solely through the OGL 1.0a then wouldn't licensing OGC via a completely new license (not just an updated version) mean they are no longer granting rights to use the OGC solely through the OGL 1.0a?
Yes. Like I set up a shop in the front of my house, and say that I will only sell for cash. Then, later on, I decide to sell also for cheques made out to "cash".
It's their statement of offer, not a term of the contract. The contract is constituted by the OGL, which does not forbid someone offering a different licence in respect of their work. (Of course that alternative licence couldn't purport to be
exclusive, if the work has also licensed under the OGL (and hence) on an irrevocable basis.)
Section 2 of the OGL mandates a statement similar to that one be added to all OGC. Doesn't that make it a term of the OGL?
Section 2 requires a notice "indicating that the Open Game Content may only be Used under and in terms of this License".
The SRD does not contain a notice literally in that form. It contains the statement of permission to use solely under the OGL; then three paragraphs down there is a long list of items that "are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content". Then there is a statement that "All of the rest of the SRD5 is Open Game Content as described in Section 1(d) of the License."
Of course, WotC do not need a literal section 2 statement, because all of their material is OGC only in virtue of their voluntarily declaring it as such, by making the offer to the world to license it under the OGL.
Whereas when someone else publishes a work that includes OGC authored by WotC or some other party, they are granted a licence to use that OGC only in virtue of the operation of section 4 of the OGL. The section 2 statement made by that publisher ensures that they expressly state the terms on which they are licensed to use the OGC and to sub-license its use.
If that publisher has authored their own material in respect of which no one else has a copyright claim (ie it is not derivative of another's work), then my view is that nothing would stop them from simultaneously licensing that material pursuant to another agreement, should they wish to do so.
An example:
Suppose I write up a set of RPG rules for determining when encounters happen in a fantasy city. And suppose, for the sake of argument, that no one else has any sort of copyright claim in the rules that I write up (ie they are not derivative of Gygax's Appendix C City/Town encounter matrix, nor anyone else's random encounter tables, nor Vincent Baker's Oracles in In A Wicked Age, nor the Events tables in Torchbearer, etc).
I could publish my rules without need a licence from anyone. Having done so, I could also licence you to include the rules in your publication.
Assuming that my licence granted to you did not include a binding promise that it be exclusive, I could also publish my rules,
including stat blocks from WotC's SRD, pursuant to the OGL, and if I wanted to I could declare my rules OGC. (I don't think I would be obliged to do that, although I will admit I find the definition of Open Gaming Content in the OGL a bit hard to parse.)
At that point you would still have your licence, but others who take up my offer under the OGL would also have licences. And they would be authorised to sub-license, although (as per section 2) only pursuant to the OGL.
In the scenario I've just set out, I don't see that I am breaking my obligations to any licensee.