I have an old Ontario pilot knife with a nylon sheath that's been used almost daily for over 3 decades. Dirty and stained with sweat, mud, manure, blood, gasoline, diesel, Jet-A and God-knows-what-else, but still in fine shape. I don't see any reason why it wouldn't last as long as the knife unless you did something stupid (burn,cut, etc). Kydex sheathes will probably still be around for alien archeologists to dig up long after humans are extinct. Other than the few thousand head of cattle behind the house, I don't have much leather around here.