#4 Nothing left to prove
"The Streak" has been one of the hallmarks of WrestleMania and has also helped WWE make the modern day WrestleMania pay-per-view the blockbuster it is today. Without "The Streak" and The Undertaker's showmanship of old, there simply isn't a reason for The Deadman to have another match.
Sure, WWE could stick The Undertaker in a match against John Cena or even have Roman Reigns Part II, but what would that prove? What would that even achieve, in the end? The only reason for a decision like that is selfishness on the part of the fans and hopefully they realise that The Undertaker has absolutely nothing left to prove.
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Consequences aside, a match with The Undertaker isn't going to elevate anyone anyway. It's going to anger fans and make WWE look bad for humanising one the company's biggest stars. Just imagine what another loss to The Undertaker's streak would do, and how it would hurt his legacy moving forward.
Furthermore, what could a win do? It might give The Undertaker one more victory and one more WrestleMania moment, but does he really need that anyway?