See above. But also deliberately just asking stupid questions to try and be derisive.
You seem to be the only one setting some arbitrary requirement that specific threats require specific classes to counter that threat, which is a nonsense argument.
Using the Legion example again: do you think that on the other planets that fought the Legion, that the only classes/ people that fought against them were Demon Hunters, Warlock, Paladins (Army of the Light) or Mages (Guardian)? Other planets, other races, with completely different cultures would use the exact same classes to fight the Legion? What if they didn't have those classes listed above? Would they just willingly bend over and take it?
FYI, that's meant to be rhetorical because the obvious answer is: NO. They would use whatever classes, weapons, tactics etc... they could to defend themselves without being restricted to some stupid arbitrary class requirement.
Now extend that logic to Azeroth. Are you implying that the only class that's ALLOWED to fight against the Legion are Demon Hunters, Warlock, Paladins (Army of the Light) or Mages (Guardian)? Why can't Tinkers, DragonKnights, Necromancers, Dark Rangers, and whatever else other class that might exist, literally not be allowed to fight the Legion?
Demon Hunters obviously fit, that doesn't mean other classes don't.