Okay, we need to sit down and talk. It's time to accept the fact that there is no way Shinsuke Nakamura vs. AJ Styles is main-eventing WrestleMania. Yes, Shinsuke won the Rumble, and yes, it's clearly going to be match of the night, but when you consider how Roman Reigns and Brock Lesnar are pushed as the top guys in WWE, it's clear they're destined for the main event slot. Heck, I'm gonna be a pessimist and say Ronda Rousey's tag match and Undertaker/Cena are gonna get higher billing.
Still, when you're a nerd who writes about way too much about wrestling on the internet (hi), Shinsuke vs. AJ is the main event, regardless of its spot on the card.
And that's not necessarily a bad thing. Despite the cache the last match at 'Mania has, that doesn't mean every match that goes on last is the best or even the most important. Throughout the years, WWE has spent way more effort in promoting a match that went in the midcard, or a match that ended up stealing the show so much, the main event was doomed to crowd apathy.
Just because you go on last, doesn't make you the main event, especially in these cases.