Matt Thomason
Hero
Using their material, since it uses all the terms of OGL 1.0a is a license between the 3PP creator and someone who wants to use their work. By agreeing to 1.0a, they have agreed that their work is forever usable by others via section 4 of the OGL. That doesn't go away if 1.0a goes away. Only WotC and it's agents can deauthorize or create a new OGL.
Forever usable by others via the OGL. You have to accept the OGL and follow its terms in order for that contract to be formed. If for some reason you find yourself unable to legally place the OGL in your book, you can no longer use it to reuse things licensed out under it, because you will no longer have a license to use that content. Not having a license to use it means your use of it will now be a copyright violation.
The only thing giving you permission to use the content is a contract between you and the creator, called OGL 1.0(a) (unless the creator did something silly with their Open Game Content declaration and gave you permission there)
The OGL is not a grant of permission. It's an offer to enter into a contract under the exact terms listed in the license.