If there's only nobles then the nobles will start eating out one another and that's a no-no, and real shit can start if nobles start fighting one another.
Having a commoner class gives a simple and easy fix to that issue by giving them "acceptable" targets to play with and have funsies and abuse.
Nobles are pampered? Of course, that's the whole point. Magic is declining? Who cares as long as they can keep up having the fun easy life. Stiffing the commoner potential? lol imagine caring about commoners lmao if you like commoners so much why don't you kiss them?
This is society dynamics 101. You don't even need need magic to see stuff like this or even worse happening IRL.