#2 Lesnar's winning would send a wrong message
From 2008, WWE's target audience shifted significantly from adults to kids. Slogans like "Never give up" became an instant hit among the kids. During the whole rivalry between Brock Lesnar and Roman Reigns that spanned over six months, the whole feud revolved around how the hard-working talents on the roster get ignored for a self-centred part-timer like Brock Lesnar.
The whole point of the feud was to demonstrate how "hard work" and "determination" can help you overcome any obstacle.
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On the one hand, we have AJ Styles, a hard-working champion who shows up day in and day out and on the other, we have Brock Lesnar, who shows up only when he feels like coming. The dynamics here is apparent. Lesnar's winning would send the opposite message to the target audience of WWE.
Lesnar's winning would even send a wrong message to rest of the locker room. You could work all days in a year but you would be always treated inferior to someone who came from UFC.