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.
How could it hurt Punk or Cena's career in the long run?
First of all the streak is impressive entertainment wise but don't forget that this is all pre planned. Taker is what Hogan use to be to the WWE, in fact i would go as far to say that he is a notch higher than hogan in a few ways. Takers streak should not end as this is what it was suppose to be for a very long time now and i agree with someone else here that if it were to end the only superstar that should end it should be his brother KANE. Kane is a joke these days and i wish they would make him darker like back in the days when he first arrived on the scene.
Kane of all people? Why? Him being the top heel in the company for months to come after that Mania would make 0 sense. They could run the storyline big time with someone. Taker vs Kane has been done too death. I'd rather see Cena or Punk end the streak.
Honestly I vote yes. But only if it is done right. And by right I mean the person who beats him at mania has to be someone the company can trust not to walk anytime soon and someone they want to be the biggest heal ever. And really the only person I can see doing that at this time is Punk. Not because I am a Punk fan its just that he is the only one that makes sense to me.
IF taker loses at mania the one that beats him will be an insta heel. That person will get booed like crazy. And since alot of us like to think that WM 30 might be when he finally hangs it up it would make sense for him to face Cena. So he loses to Punk this year and gets his one last match next year and beats Cena. This gives Punk crazy heel heat and instantly gives him something to hang over Cena's and everyone else head. He has beaten Taker at Mania, which no one else has been able to do.
I think Cena beating Taker would be the biggest payoff because everyone wants him to turn heel. The only reason people boo Punk now is because Heyman is with him.
I'd say yes, but only on the condition it's someone young and talented, who like others have said, can be the future of the company. No Ryback types.
If built-up right: Roman Reigns, Dante Dash(relatively unknown but I'm a huge fan), maybe even Richie Steamboat. Other than that, I just don't see it ending. But if it does, then it become Undertaker's retirement.
Honestly I think the streak should end, but only under one condition. Whenever his last match is it should be a last man standing match and end in a double knockout where taker never actually lost at mania but it would end the streak.
I said No to Taker losing @ any 'Mania let alone his last. Losing and ending the streak, even for retirement, would ultimately tarnish his legacy and make him no better than any other wrestler who has ever laced their boots. Nobody ever remembers a streak in good terms once it ends, the fact that the streak did end is at the forefront of people's thoughts and opinions, over-shadowing everything that has happened previously. Like it or not it's the truth, and that's life.