This is a good question and I have two answers, yes and no.
The Undertaker should never be beaten in a WM match by anyone on the current roster. I think right now anyone beating the Undertaker would be a bad move. Most of the people in the WWE would not be believable and the few that could muster it, Triple H, Punk, and John Cena don't need it and it would possible hurt their career in the long run.
If he lost to the right person at the right time. Right now there is not a right person and the right time would be Undertakers last match.
So right now the NO has it unless they can find that right person. The next Rock, Stone Cold, or Y2J even but right now that person is not with the WWE.