#2 It would take an opportunity from younger talent
The obligatory yearly Undertaker match would be taking the spotlight from younger talent that could put on a better show. It's that simple.
Currently, Elias, Braun Strowman, Rusev, and more don't have an obvious WrestleMania match, despite how over they are. Each of them would put on a better show than an Undertaker vs. John Cena match.
That opportunity cost has been a persistent problem in WWE for years and this time would be no different. Not only would an Undertaker vs. John Cena match take the spotlight from younger stars who could use it, it's indicative of a long-term problem of the promotion's inability (or unwillingness) to create new megastars.
Elias could use the opportunity to get in the ring with John Cena at WrestleMania far more than Undertaker could, as could Braun Strowman, who would be likelier to win.
Say what you will about Triple H's WrestleMania matches, but at least he's been giving spotlights and rubs to younger stars. Only this year does this streak of his appear to be ending.
John Cena has been the opposite. A match with Undertaker would only put an exclamation point on that.