For me, it's because if either WotC or a company who bought WotC abuses this clause, I would simply stop buying product from them. And with the core rules existing in CC and apart from any one companies control, I'm sure I could find another system using the core rules of D&D from another company. For example, I recently deleted my Twitter account due to the person who bought that company.
Ultimately, WotC's right to enforce that provision as defined in the proposed OGL comes with it an implied responsibility to use it wisely or end up losing customers. Let's say, for example, that a company buys WotC and determines that Critical Role is in violation of the clause as a way to shut down their production of their D&D related supplements. What would the reaction be?