#4 He's already hinted at retiring a few times
There are two stars I'll refer to when it comes to retirement and WWE. The first is Ric Flair. He lost a match to Shawn Michaels at WM 27 that required him to retire had he lost. The Nature Boy did retire from a WWE ring in theory but he then joined TNA/Impact and wrestled a few more matches. It lessened the impact of his retirement in WWE and the ceremony dedicated to it.
Michaels suffered the same fate as Flair when he put his career on the line against the Streak at WM 26. The Undertaker won and the Heartbreak Kid did not have a match for eight years. He then teamed up with Triple H against Kane and Undertaker at Crown Jewel in 2018 but has not competed since.
My whole point is that the Phenom has actually teased his retirement a few times after big matches. The first was in the picture above after he defeated Triple H in a Hell in a Cell match at WM 28. The three men helped each other up the ramp as we fans witnessed the end of an era. While that was almost eight years ago, the Undertaker has competed regularly each year since.
The second time he teased leaving the ring was after his loss to Roman Reigns at WM 33. He left his gloves and other gear in the ring after departing. That's a signal that a fighter is hanging things up. If he truly decided to hang things up after facing Styles, no one would fault him. Since he's already hinted at doing so before, it wouldn't be a surprise if he does so permanently this year.