#1 Interference from Shawn and Kane, but finally Triple H prevails
Triple H has been losing almost every singles match he has been in recently (with the Jinder Mahal exception). One thing Shawn was right about was that Triple H sure has more left in the tank than The Undertaker does.
He is younger and in better shape, he is therefore likely to go on for much longer. If he beats The Undertaker, he will restore his credibility as an ace performer. Therefore, future stars who get the better of him will have something really big to brag about.
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Also, because he sort of cheats to win, The Undertaker's credibility does not diminish that much. Yes, he does not come out as invincible as he once was, but that is good in a way. It could ensure storyline continuity. Therefore, I feel it's a win-win situation.