No, what I stated was that fireball is exceptional against multiple weak targets. I mean, are we acting like it's news that fireball is kind of OP?
Most other AoE is more balanced, either because it's weaker or trickier to position (i.e. no one complains about lightning bolt, even though it does the same damage as fireball). But sure, I agree that casters tend to be better at AoE than ST (again being cautious, since this argument tends to constantly devolve into fighters and wizards, without even accounting for subclass). More because of control than damage, IMO; a cleverly used web spell can be encounter changing.
A clever use of web can be encounter changing, even at high levels. So can a hold spell, or force cage, or literally any control spell. Which brings us to this.
Yeah, legendary resistances are aimed right at save or suck spells. The point being? Legendary resistances are very much a thing in this game that spellcasters have to deal with. Good thing there are no legendary resistances to "great sword to the face." For that matter, there are all kinds of other resistances that mostly affect spellcasters, especially at high level play.
Do you know why there isn't a legendary resistance to Greatsword to the face? Because no one cares. The solution to fighters is literally just giving the monster more hp. That's it. But more hp doesn't matter in the face of casting. The only way to keep boss monster's relevant, was to give them an anti-spellcasting ability.
The example given was a dragon battle. We've all probably run our fair share of those. Given the options offered, I stand by my argument that I would rather have two fighters and two rogues rather than two wizards and two clerics (though obviously one of each would be more optimal, which is the design intent).
Edit: Hmmm...mathing it out...I dunno. I think for sure the fighters are excellent in the dragon encounter - high DPR and survivability. The rogues are a lot weaker. With the clerics hopefully tanking the wizards could try to focus on burning through those resistances to get off a lucky polymorph on the dragon or something, though the problem will be surviving long enough to do so - dragons are smart and mobile and can feasibly kill a level 15 wizard (probably, what, 75 or so HP?) in one round, two on the outside unless the wizards are focusing their magic on survivability, but then it's a matter of diminishing returns - can the clerics do enough damage to get the dragon off the wizards? Probably not.
Thinking about the encounter frankly makes me more impressed with 5e's design balance.
Fighter's are excellent in the dragon encounter if they are archer fighters and designed to fight the dragon.
Wizard's and Clerics are able to take down the dragon as long as they know they are fighting a dragon. Like, seriously, your entire point is "will the wizards and clerics live long enough to win" and if the clerics and wizards know that they are fighting a dragon, the answer is yes.
I mean seriously, this is kind of even a ridiculous question. Let's make the most powerful fighter's we can with their most powerful options. Archer fighters with sharpshooter, magic bows, the works. They are going to fight the dragon in a knock down drag out fight. I mean, the fighter's can deal... 1d8+18 = 22.5 x 3 = 66.5. +13 to hit, -5 = +8 vs AC 19 is 50% accuracy. 66.5 x 0.5 = 33.25 per turn per fighter. Rogue goes in with... well, nothing. Can't exactly sneak attack. So their contributions will be minimal.
If I don't max out the Dragon's hp (using an Adult Red by the way) that is about 4 turns of combat, let's say three turns with Action Surge and special abilities. On average.... assuming the Dragon Fear doesn't hit them. Because dragon fear would lower their accuracy. But the fighter's can re-roll that DC 19 wisdom save, so I'm sure they will make it right? And the dragon's breath if they fail the DC 21 save is about 50% of their health, so I mean they'll likely be at a 75% health after that, and then dealing with the Dragon's attacks which could finish them in... Well, the dragon does about 53 damage a turn and the fighter has about 139 hp. So... Might be close. And if I increase the Dragon's hp.... that could go very badly.
But surely the wizard's and cleric's do worse right? Let's give them their best tools too.
Well, we are working with this number of spell slots 4/3/3/3/2/1/1/1 per character, which is 16/12/12/12/8/4/4/4
Four Death Wards will prevent instant death. Then both wizard's cast Simulacrum, giving you two additional wizards. That gives us another 8/6/6/6/4/2/0/2 spell slots. Heroe's feast makes them immune to dragon fear and increases max hp. Can probably get away with some 5th level Aid's? That's more max hp. Don't want to do concentration from protection from energy, but I can have the wizard's simulcrum cast invisibility at 5th level. Sure, Dragon's have blindsight 60 ft, but they have normal sight farther, and we just need to get close
Man, used a lot of spells right, we are down to...
24/18/18/14/9/5/2/6
huh.
Clerics still have a mass of healing spells. And some things like banishment. Wizard's can wall of force to keep the dragon pinned. If they get really lucky with the initiative that alone could trap the dragon into a blade barrier and wreck it. Or, what about throwing four feebleminds. Damage is low, only 16d6 or 56 damage (about 1/5 of its health) but if it fails any of those intelligence saves (at a +3 vs DC 18 that is a 70% chance of failure) then their INT and CHA is reduced to 1.
Charisma saves become a mere +1 vs DC 18 (80% chance of failure) against things like banishment to keep the dragon from attacking the party, and their intelligence saves become literally unpassable. Allowing the wizard's to hit it with Mental Prison multiple rounds. That is 5d10 damage, restrained, then another 10d10 damage if they attack out of it. Which they will, because 1 Intelligence. So, that is 15d10= 82.5 x 4 = 330 damage...
So the dragon's only chance would be to kill the wizard's in round 1... which it can't. And then get lucky with the feebleminds, which at a minimum likely eat up all its resistances, and then avoid...
Sure, this required specific spells, and it isn't like the wizards are guaranteed to have these spells. But... This also isn't unreasonable. I didn't really name any spell people typically avoid. And you only need one of the wizard's to get the spell for both to have it. And... other than redesigning the dragon... it can't win.