I agree, I don't see it happening either, and kayfabe deaths are very problematic, as the kayfabe is so thinly veiled by now.
And don't get me wrong, I love Undertaker as well. But even though he's already given away the streak and given Lesnar a huge push, to have him go out with a loss at Wrestlemania in Texas will pretty much make anyone who beats him a legend right then and there. So I think it's a wonderful way to give someone in the mid-card a huge push. To broaden the roster of real superstars. And I want that to happen to Wyatt. All in all, I think the best and most legendary way to go out is to pass on the torch, of sorts, to a new guy. I think his reputation will be even greater in some years of he goes out like that.
Anyway, you're right. He also reminds me of early Mankind, but even more deliberately sinister. And that's awesome. Undertake is up there, but Mankind is my favorite Wrestler of all time.