His vocals aren't easy to replace at all, his has a multitude of very unique and specific vocal inflections that appear all over his work.
Having a way with words is a given, but kendrick can achieve a perfect balance of cadence and delivery, his bars are nothing to snuff at either.
But the most important element that's irreplaceable is the fact that he is who he is.
He has such great fan loyalty because his music is really personal, and listeners are charmed by his character, history and personality. He really lets you get in his head and so people can really really connect with him.
Typically artist, sing/rap out of character, or the song isn't about them and more about material things, or a concept/idea.