Specialization is very much a thing in US sports as well, even without relegation and even with profit sharing/etc.

The payoff to being elite at a valuable skill is enormous. Teams generally benefit more from combining players with distinct, elite strengths than from relying on broad generalists who are not truly elite at anything.

This isn’t always possible if you can’t afford to build a team of specialists, or those specialists don't exist at your level of competition. But if you have the resources and coordination (and in sports, the roster depth and cap space) to cover each specialist’s weaknesses, specialization is pretty much always the stronger composition.