Mmm, but you're forgetting; the GM isn't having to balance around only one character. What you're proposing is essentially restricting the effective uses of magic to "buff allies" and "shoot things at people." Again, we have melee characters--in fact, in a multi-character roleplay it is completely implausible to wind up with only magical specialists or the like.

One character who can negate all magic to themselves and one character good at dealing with, essentially, brute forced buff melee or physical ranged attacks would negate all direct threats. Out of those proposed, only indirect illusions and environmental effects remain and trying to construct an entire roleplay out of those limited circumstances [i]is[/i] a headache. Plus it completely undermines what is supposed to be a high magic scenario where characters are really, [i]really[/i] good at what they do and specialised by default.

It's worth noting that complete magical negation barely ever makes it into RPG rulesets. Out of many editions of Dungeons and Dragons, I think only the Iron Golem gets away with just shrugging off literally any magic you might put in its general direction and it's an intentional slow and hulking mage killer. Other magic resistance can be pierced and overwhelmed (whether it's the monster's toughness or the caster's ability depends on the edition) and the cases where something BENEFITS from absorbing magic are extremely limited. Doubled by the Beholder being one of the most fearsome enemies for all its absurd appearance because it spams magical negation and magical attacks in tandem. Strong and drawback-free magical immunity wildly imbalances any magical setting to an impractical degree because it restricts magic to doing things you could already do with just a bigger monster.