The player requests that demons be made irrelevant: someone else seals up the rift to the Abyss, someone else beats up the demon lord, and these two off-screen victories by NPCs come with no meaningful fanfare. If there is a sudden, epic showdown with the Abyss, NPCs should get the job done instead. The player just wants demons done, gone, and never mentioned again.

Yeah, no. At this point it's obnoxious and insulting to the GM, demonstrates a complete lack of regard for both the GM and the other players and they can knob off, frankly. Reeks of "I'm the main character" in real life.

I'm sure some will find this a bit blunt. That's fair enough, and maybe I'm spoiled by only ever playing with friends. Still, it is staggeringly obnoxious, especially so far into the campaign.