#2 It will benefit his future opponents

As an authority figure, Corbin had a ton of momentum. I agree that it wasn't because of his great work, but what he did as a villain. However, since losing his position in WWE, Corbin has fallen down badly.
Even though he has defeated Angle on many occasions, he still doesn't have a good amount of momentum. Considering Angle is going to retire at WrestleMania, there is genuinely no other way WWE could generate serious heat for Corbin if he didn't retire Kurt at 'Mania.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
I know the fans will boo him badly after WrestleMania, but it is what Corbin wants. If he receives one of the loudest boos, then all it means is that Corbin is doing fantastic work in WWE. The company could then pit Corbin against some top babyfaces who need attention. This way, Corbin will be needful in future booking decisions.