#1 No need to tarnish their legacies further
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The reason Sting didn't come to WWE in the early 2000s was that he didn't think WCW stars were being treated properly in the company. Sting eventually debuted in WWE, and went on to lose to Triple H at The Show of Shows, and later to Seth Rollins. WWE won't, and rightfully so, let Undertaker lose to Sting if the two ever collide. Would Sting really want to look back at his WWE career and think that he made a grave mistake by ending up 0-3?
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Then we have The Undertaker, whose retirement was teased after a horrible WrestleMania 33 main event against Roman Reigns. It didn't happen though, and he is still wrestling for WWE. Sadly, we're way beyond the point where we could talk about saving his legacy. The past three years have made sure of it. The best we can do at this point is to hope that The Undertaker's legacy isn't tarnished further with another embarrassing match.
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