That's just a phrasing issue.
"I'd like it if your other players could share the spotlight a bit more. What I'm planning to do is reduce some of the power in your intended class's spellcasting. You will still be able to cast as often. You just won't have as much access to spells that can single-handedly end a fight."
And then you proceed with normal rational dialogue and negotiation. Maybe you go through with it and maybe you don't, but resentment isn't a necessary outcome, and, in a game played with friends, it doesn't seem that egregious to give up some amount of power so that some of your friends have a little more time to shine.