Let's assume there is a project under GPL2-or-later license. My understanding is that anyone can take the project and make a fork with GPL2-only additions. They could also take the project and make a fork with GPL3-only additions.
Now, what if a company wants to make a proprietary fork? They can take the project, and make half of their changes under GPL2-only and the other half of them under GPL3-only. Since those halves would be a derived work, normally it would infringe, however since the copyright holder is the same entity, they would not sue themselves.
Now, in addition, they could add into their customer contract a binding promise not to sue them for the violation as well. The result would be that no one except their paying customers can legally use the final product.
Now, this is a loophole so obvious, that I am sure it is not possible. But I would like to understand why it is not possible.