#1. Burying younger superstars
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When Brock Lesnar does continue his run in WWE, there's a very high chance that he will be taking on a fresh set of opponents. He's clearly done with The Shield members, all of whom he's feuded against. There's no real incentive for WWE to go back to Braun Strowman-Brock Lesnar.
So that means that he'll get a fresh set of opponents. While that should be a good thing, it really isn't at all. You can bet that Brock Lesnar won't be putting in a 100% into the matches and they'll likely end quick and they'll also most likely end up in victory.
Sure, a Brock Lesnar win is reserved only for superstars of the highest level like Roman Reigns and Seth Rollins, but even so, his matches do no favours to the people he faces, because they don't come out looking better. The only two exceptions in recent memory are AJ Styles and Daniel Bryan, but otherwise, it provides no benefit.