The problem is giving up performance for the current overwhelming majority of computers. Slowing down YouTube slowing down for IPv4 computer won't create an incentive to upgrade those computers. It will be an incentive to leave YouTube for a faster competitor.
This strategy requires a whole group of players to move simultaneously. We're back to the chicken-and-egg problem.
It already does for other criteria, like how much is paid by the content provider to have the relevant cable "still work flawlessly" (such behaviour sometimes raises a scandal).
The problem is the network you talk about is a bunch of interconnected machines and cables, each under the control of some entity, which is often a profitable company. What you propose would be very difficult if you want to preserve (the illusion of) net neutrality.
This strategy requires a whole group of players to move simultaneously. We're back to the chicken-and-egg problem.