Btw. your example is missing the bus factor. But I understand your point and actually I do not think that salaries are meant to depend 100% on performance.

I think it is more like 90% of time the person invests and 10% performance bonus. That way the one with to 50% higher salary would probably be 5 times as performant as the average dev. If that is fair, is a different (and very difficult) question.