Locke didn't die because he was missing a kidney. If he had the kidney he would have died.
that'd be a neato tie-in, but i don't think that's (the only reason) why he didn't die. locke regained the ability to walk on the island, had his shattered legs heal super quickly more than once, and came back to life in an even more dramatic fashion in the second plane crash.
clearly, the island wants him alive.
