FlyingChihuahua
Hero
Higher level Fiends like Pit Fiends and Balors can ruin days pretty well without using magic. Though Pit Fiends have the same problem as Lycanthropes, in that they have weakness to silvered weapons. Not Balors though, they have resistance to non magical damage.Yeah, since silver weapons work against them, I didn't even bother with them.
Powerful creatures which even have resistance to non-magical weapons still typically rely on magic or magical features to make them actually powerful IME. There might be a few exceptions, but none I can think of offhand.
Hell, a decent amount of extraplanar creatures have resistance to nonmagical damage. Most Demons do, most Yugoloths also have it, Elementals (but not genies because ??????), incorporeal undead like Ghosts, higher level Fey, and other miscellaneous things like that. Granted, most of that is Resistance and not Immunity, but it still helps them survive, and a lot of them can still decently go toe to toe with the more martial members of the party at their CR even without magic, which would leave the squishy casters in a world of pain. I'm remembering that one Order of the Stick strip where the elf wizard went off to fight a dragon, which proceeded to cast Anti-Magic field and gloat about how the wizard just ended up being a squishy monkey while the dragon was still a dragon.
But again, this is all going back to setting up an anti-magic field in the fights where the Wizard would stomp it without a second thought. For one, setting up that many magical dead zones is just lame and reeks of "I'm specifically targeting this one party member" which is a really good way to get that player (and the others, probably) upset with you and it also is just a dumb way of doing things. Like imagine if the Fighter started being a problem on this level, so before every major fight where they could be a problem, you threw a bunch of Rust Monsters at the party and had them focus exclusively on the Fighter. That's just dumb and lame.