#2. By destroying Seth Rollins' credibility as a competitor.
In WWE's dictionary, heels should not cleanly defeat babyfaces. Of course, there are a few exceptions like Brock Lesnar and The Fiend. But on a larger scale, WWE is not keen enough to build the credibility of heels. The biggest example of WWE's approach is the current situation with AJ Styles. On the road to WrestleMania 35, Styles had a feud with The Viper Randy Orton. Styles portrayed the babyface and Orton portrayed the bad guy. Styles won the match and even kicked out of an RKO.
Fast forward a few months and AJ Styles is a heel with Luke Gallows and Karl Anderson always looking out for him. I cannot recall the last time Styles won a match without interference from Anderson or Gallows.
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If there is anything wrong with Seth Rollins' run as the WWE Champion in 2015, it is the fact that WWE never took Rollins seriously as a Champion. Rollins had to cheat in most of his matches and even lost most of his non-title clashes. J and J Security, Jon Stewarts, The Undertaker - the list of people who helped Rollins retain his title goes on.
WWE should not repeat the same mistake now - especially since WWE invested a clean win over Brock Lesnar on Rollins.