I imagine our hard limit is human attention. We can focus on the lead and to a lesser extent listen to the background stuff but probably not listen to much more. I don't think we can really focus on some large complexity. That just ends up sounding like a discordant cacophony to us. If anything, we're made to enjoy fairly simple musical relationships. Chords can only get so complex before they sound off to us.

It seems to be a happy little coincidence that ten fingers is enough to make complex music without hitting too many practical limits. Just because that limit exist doesn't mean its wrong and that we need to overcome it. The same way a third leg won't let us run faster, fight better, or climb better.