That's an interesting question: would anything change if the average NFL salary was $500k instead of the almost $2m that it is now? My guess is no.
But let's say you are 15 year old super talents that has every potential to make many millions in the NFL today. Would they still pursue football if they could only make at most 25% of the highest paid players today or would they focus on another sport, and if so which sport might do better in this future? Perhaps baseball or soccer will become more popular since if you're not getting paid a lot why risk the head trauma.
Some would argue that that bit of economics is part of what is diluting the product on the field. There is little incentive to keep mid-career, mid-talent guys around, when you can use guys on rookie contracts that are half or a quarter the price, for 75% of the skill. Except there's a lot of deficit in experience and savviness.