It would be cool to have an everstone item object.
"Mash B to prevent belt graduation!"
But yea, these people here answered it pretty much. It keeps you in the right skill level ingame. Sure, you can keep creating alts over and over again to keep fighting white belts but where's the satisfaction in that? Eventually you'll just want to progress and when you do you won't even have improved since you were fighting white belts the whole time.