Probably true but how many devs that think they are 10X are just regular devs with 10X ego that compare themselves with the lowest possible dev to feel superior.

I could see a union that won't put upper limit but would fight for fairness, extra hours paid double, no more then X hours a month etc.

Except that the very concept of a 10X developer, no matter what they themselves may say about it, is disgusting; it's a person who has, even if willingly, distorted their work/life balance to a point where they "go the extra mile" as management would put it; usually involving working off hours, working extra hours, working from home on weekends and evenings, or eschewing general life activities in favor of doing more work.

I know this is sacrilege to say this in our system and even moreso in our profession, but this is not good for you. I don't care how much you like it, I don't care how much money you're gaining from it, I don't care if you're the most willing of willing to have ever willed. You're abusing yourself in exchange for money, and that's the end of it and it shouldn't be allowed.

You should have a salary commensurate with your skill set, and you should not have to eschew anything resembling a proper work/life balance to do it.

You are assuming that greater value generation is only a function of greater effort in the form of more hours. It is also a function of other things like higher talent, smarter decisions, and past investment in one’s skills.