I disagree. Or rather, I agree in theory, but it can be much more difficult in practice.
Whenever you see a group of creatures, you probably assume that they're weak. (The DM wouldn't throw a bunch of strong creatures at you, all at once, right?) So you throw a Fireball at them, expecting to kill them all. If they don't die, then you've just revealed itself as the biggest threat on the field, and now every single one of those enemies will be pointed right at you. This not being a game of battlefield control and tactical maneuvering, you're probably going to die now.
A couple of things make Fireballs much less useful against grouped foes than other editions:
1) Bounded Accuracy means that weak foes can still kill you. If those orcs happen to be Berserker NPCs (CR 2), then they will slaughter your level 10 Wizard.
2) Hit Points scale faster than they used to. An enemy with a slightly higher CR will have a lot more HP. You have no way of knowing, beforehand, whether your Fireball will be a one-turn victory or a waste of an action.
Put together, it means area effect spells are a huge gamble. Sure, it might work, in theory. Your enemies know that, too, though. There's no reason to expect it should work in practice.
(Thanks for engaging! Let's see if I can come up with anything interesting to say in response to your interesting post.)
Hmmm. Some thoughts:
1.) Bounded Accuracy applies to AC. Wizards have non-AC-based defenses like Blink, Invisibility, and Expeditious Retreat/Dimension Door.
2.) Berserker NPCs are actually weaker against wizards than orcs are, because orcs move faster and have a missile weapon. A wizard can just turn on Expeditious Retreat and outrun the berserkers; that doesn't work against orcs.
3.) 95% sure you were joking when you said "This not being a game of battlefield control and tactical maneuvering, you're probably going to die now," but just for completeness: manipulating the enemy into believing you're the biggest threat on the field functions as a defense for everyone who isn't you. If you can chuck a fireball and vanish behind total cover, that gives any archers on the field free rounds to shoot at the enemy while they're chasing you. Possibly a
lot of free rounds.
4.) You can surround yourself with summoned elementals and/or animated undead prior to casting. Then overcoming your defenders will require a critical mass of enemy troops (because: melee version of the artillery equation means that effectiveness is roughly equal to quantity squared, so 2 orcs will die horribly to your four skeletons without accomplishing anything), and Fireball is ideal for dealing with critical masses of troops.
5.) Hypnotic Pattern is also an AoE, and unlike Fireball it can disable berserkers about as easily as orcs.
6.) If the 12 orcs you see all turn out to be full orcs with the aggression trait
and Berserker stats, and keeping the range open isn't an option, your whole party is probably dog meat anyway at close range. Bombarding the enemy with Fireball/Hypnotic Pattern can't possibly make the situation worse than plinking away with single-target spells like Magic Missile. Either way those orcs are going to be chopping away at
somebody.
Conclusion: AoE spells are only a gamble if you're putting all your eggs in one basket and hoping to kill the enemy in one blow. However, because AoE spells generally don't take concentration, they are fully-compatible with a combined-arms approach that mitigates the "orcs all turn out to be 67-HP berserkers" scenario unless the enemy has overwhelming force, in which case you must run or die regardless of Fireball/Hypnotic Pattern antics.
