The OGL is a contract. Both sides have to agree to the severability of a contract. In general, one party cannot simply declare a contract you have agreed to invalid.
Section 9 doesn't grant Wizards the right to do this either. That clause is about updating the license. It's a way to opt-in to a new contract, not a way to sever the one you've already agreed to. If Wizards were to make OGL 1.0a an unauthorized license, it would mean that you couldn't relicense other OGL versions into that one anymore. Each of those things are still licensed under their original terms.
The reason they want to do this is because they screwed up and didn't say that you can update to a later version. I was shocked to see that the upgrade section didn't contain this. Saying it that way is how the GPL gets around the idea of people "downgrading" the license in order to do something the original licensers don't want.
Having said all of that, RPG companies regularly claim all sorts of rights they don't really have. They often pull a sort of "protection racket" move where they coerce you into a contract with them by threatening to sue you in a way that will not hold up in court. It's not really surprising to see them trying to do this with a new version of their license.
