#2 Could have been kept alive in the cinematic universe
One of the biggest problems with The Undertaker's retirement is the fact that it comes right after WWE showed they could protect his aura through cinematic matches. In fact, between the thrilling supernatural elements of The Boneyard Match at WrestleMania 36 and how it protected The Undertaker's weaknesses, they could have done so many more matches like this.
Think about it! The Undertaker looked awesome during his match against AJ Styles and could have had many more cinematic masterpieces if he wouldn't have announced his retirement. He could have even had that dream encounter with The Fiend, which would have a feud for the ages.
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If nothing else, it's just so disappointing to see WWE introduce a whole new universe for The Undertaker to exist in and then make it a one time thing.