The open license will always be valid for any products published while the open license was open. WotC can never take away Pathfinder, or Tome of Beasts, or anything like that, even if the license is revoked. But if the license is revoked, those companies could not publish any new material as of the date it was revoked.
This is the bit that confuses me (as a non-lawyer).
The core part of the OGL is this:
4. Grant and Consideration: In consideration for agreeing to use this License, the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to Use, the Open Game Content.
"Use" is defined in the OGL as "to use, Distribute, copy, edit, format, modify, translate and otherwise create Derivative Material of Open Game Content."
"Distribute" is further defined to mean "to reproduce, license, rent, lease, sell, broadcast, publicly display, transmit or otherwise distribute"
So, when Paizo released Pathfinder, they used the OGL, and were thus
granted a perpetual license to license Open Game Content. So given the license's viral nature, I should be able to rely on e.g. Paizo's offer of the license instead of Wizards'. And there are a few places that have copies of the SRD published without any changes.
And this license is printed in quite a few books, as required by the license itself. That printed license promises me that I can use the Open Game Content in that book under the terms of the license. Again as a non-lawyer, this does not seem like a thing Wizards should be able to revoke.