Rival leagues paying players in the same range as the NFL are not even remotely a possibility in this day and age, so let's do away with that silliness. No one is going to sacrifice tens of millions of dollars just to spite the NFL.
The thing no one here has mentioned is that if this ever came to fruition, the NFL wouldn't implement a QB salary cap. They would simply implement a max contract situation. Of course, in the real world, QBs are the only ones who would qualify for a max contract but that's their problem.
That was 30+ years ago and worked out so poorly for Toronto that they let him out of his deal 2 years early. Here in the 21st Century, it just isn't a realistic alternative