Quickleaf
Legend
I don't think it's a bad premise, just that it's really a matter of perspective rather than a "hard fact."I'd like to hear more about why you think the premise is bad.
They've created a type of monster that literally cannot be significantly affected by an entire category of player characters. What other reason can there be for such a move?
When you say "cannot be significantly affected by an entire category of PCs" I am assuming you are referring to casters who rely predominantly on spells which require saving throws? So...enchanters and illusionists, for example?
First, Legendary Resistance doesn't completely shut down such spells, just the first 3 or so such spells cast at it. In other words, it merely buys a monster time. A player using good strategy might try throwing out some lower level spells that require saves, before opening up the big guns, giving the legendary monster the choice between using Legendary Resistance now or biting the bullet and suffering the effect in order to save Legendary Resistance against higher level spells. Bad strategy against a legendary monster would be unloading with the most powerful spells that require saves first.
Second, a creative player can find workarounds for Legendary Resistance. For example, an enchanter could charm other creatures to do his or her fighting for him/her, and that would both be an effective strategy and wholly "playing within type." An illusionist might cleverly minimize the monster's ability to make physical contact with his/her illusions, thus denying the monster a saving throw entirely.