If they want 6E to not be usable by competitors, they can just release it under a different license than a version of the OGL. You can freely mix OGL 1.0a content with stuff licensed under a different license.
That is the essence of the practical commercial problem WotC faces with 6e; this is what this is REALLY all about.
They don't want the OGL 1.0a to apply to 6e. They most especially do not want there to be any legal basis for a competitor to offer a VTT program that can be used to play 6e.
Ordinarily, this poses no significant issue. The simple solution it to release 6e without any sort of OGL or SRD of any kind. The problem for WotC is that they while they want to change horses to 6e, they want that version of the game to still be backwards compatible with 5e. As initially premised, I have understood that the changes proposed to 6e are minor. They want this to be more like 3.0 to 3.5, or perhaps 1st to 2nd ed. 6e is not to be a true new version of the game, as 3, 4 and 5th ed each were.
The problem: 5e was published with the 5.1 SRD under the OGL 1.0a. When you maintain backward compatibility with that, you leave a door open for a competing VTT to offer a toolcompatible with your 6e game, in whole or in part.
So in the future, a VTT just continues to use the 5.1 SRD and OGL 1.0a as the basis to maintain broad compatibility with 6e. There are more than a few devils in those details, but that's the core difficulty WotC faces how to be backwards compatible in a AD&D 1st ed ---> 2nd Ed manner (almost identical, really) while shedding the 5.1 SRD.
Practically speaking, it may be quite hard to do this. It adds a dimension and complexity to all of this that the game designers and marketing people within WotC both want to be able to ignore. The designers don't want their hands tied, the marketing people don't want to go back to a blank slate, they want to squeeze more
sweet, sweet milk out of this cow. (They just want to do so in a way where they can sell that milk for a lot more money.)
That's very hard to do. So WotC has decided the best solution to this problem is to get rid of the OGL 1.0a by press release that 1.0a is no longer authorized, and creation of "OGL" 1.1.
For all of the reasons mentioned above in my "
But what about Paizo? post" this is a position that a court is unlikely to accept. WotC has clearly conducted itself in a manner over a course of decades in its interpretation of a contract that the 1.0a OGL is irrevocable. They used 1.0a again for 5e. That was a mistake that they are now trying to undo.
There are no mulligans here. I don't think that bird is likely to fly.