#2 Why Roman Reigns shouldn't retain: More matches between the three stars after WrestleMania
If Roman Reigns wins at WrestleMania, it accentuates his dominance on top of SmackDown and WWE as a whole. That would be the end of the feud between him and both, Edge and Daniel Bryan.
The latter has already had multiple Universal Championship matches against Reigns, while the former only has a few more stories he'd be able to tell in a WWE ring. However, if one of the challengers wins the title at 'Mania, it opens up a few new possibilities.
Edge and Bryan could face each other one-on-one at Money in the Bank if either man becomes the Champion at The Show of Shows. Beyond that, The Tribal Chief always has the last word. He is also intrinsically linked with the Universal title, so he wouldn't truly leave the picture.
Roman Reigns is unlikely to get pinned at WrestleMania, no matter who wins the match. If he loses without doing so, he would go on a warpath to get back the Universal Championship.
This way, the three opponents in the main event of 'Mania can face each other in different combinations before The Tribal Chief ultimately wins his belt back. He will end this story on top, whether it ends at WrestleMania, SummerSlam, or beyond.