If you were to build a system with no requirement for backward compatibility with cocoa or macOS, or even knowledge from such a thing existing, we have still moved forward three decades from not just NeXT, but the computing industry that NeXT was built to serve.

I'd assume their requirements would be drastically different, and the system they created would be drastically different as a result. The requirements aren't even easy to hypothesize, since iOS would never have existed.