Why didn't 4th Job AoE's get a cooldown?

It's because NLs and BMs and Heroes being great at bossing doesn't have an immediate effect on you, whereas a Mage/Bishop coming into your map and deciding they want it, does, especially when they can literally KS you out without effort.

This is a problem in ALL areas of the game. You cannot say "look what other players can do to me that is naughty" and say that it's broken on those grounds.

I could walk into Ant Tunnel and block all 1st and 2nd jobbers from grinding there. Any 4th jobber could. Does that mean 4th job is broken in general? No, that means that with power (forgive me) comes responsibility. Don't blame the class when someone does something evil to you, blame the evil person.

That's just simple ignorance, but, in their defence, making arms a little bit faster isn't worth bringing along an Arch-Mage without better reason, anyway.

A Mage with summon isn't THAT bad at single target either, at least compared to classes that aren't NL. They're not anywhere near the best at single target, but CRAZY fullscreen AoE plus fair single target means they can be useful. They can be useful... if they're allowed to be. They need to be able to use their fullscreen attacks whenever it is helpful (that is, arms and potentially during Zakum's adds), and they need to be able to use their summons (which can increase their single target DPS by a rate of about 1.5x, or in other words, it's roughly as effective to them as Shadow Partner is to NL, except moreso because their summon can hit adds at the same time it hits Zakum itself).

1: Any class that can train several times faster than all of the others is imbalanced and broken.

MapleStory is not balanced. Not at ANY point of this game is it balanced. Archers have a hard ass time until 4th job, Pages always have a hard time, Mages get shafted on elements, Bishops get pumpernickel for attacks other than Genesis, Warriors get pumpernickel mobility until 4th, Bandits suck balls 99% of the time and are so bogged down with attacks that it takes a pineappleing genius with a keyboard with 1200 keys on it to use effectively. Did I miss anyone? pumpernickel, man. Magicians are broken? The whole goddamned game is broken.

2: Any class that can effortlessly kick out every single other class on 99% of the maps in the game is imbalanced and broken.

Assassins in 2nd job. Magician in 1st. FP in 3rd. Who cares? When I play my Bishop, I don't walk into a crowded map, start spamming Genesis, and tell everyone there to go away because there's no chance they can beat me. Don't blame the class when a person uses their power irresponsibly. If I'm trying to take down a Headless Horseman and a Paladin walks in and destroys it, does that mean Paladin are broken for that? You heard it, guys, all classes are broken because they could all KS my Headless Horseman from me.

4: Being able to party them doesn't fix the fact that it's broken. Are NLs broken too? Yes. Does that make the magician fourth jobs any less broken? No.

Being able to party them means that EVERYONE benefits from them being strong. That's why it's stupid to complain that NL are broken. EVERYONE benefits from the faster boss runs. The only thing worth complaining about would be to be refused from the parties, sort of like how CERTAIN CLASSES are refused at bosses, or how certain pigheaded folks might not take certain classes as grinding partners because they're "too weak".

The only situation where I see a NL > Bishop is at bosses, because they will kill faster, and more efficiently, period.

What about 1st, 2nd, and 3rd job wherein a Rogue/Assassin/Hermit can drastically outdamage a Magician/Cleric/Priest? Honestly, I'm not even sure a Priest going all out with MG, SR, and Summon Dragon [on Holy-weak] could beat a Hermit of comparable strength. To put it frankly, all Magicians take a lot of additional stats from gear to be a little more stronger, while other classes, particularly the star chuckers, gain MAJOR amounts of damage for a little bit of stats.

1) A 15X NL walks in and KS'es Skelegons.

2) A 15X Bishop walks in and KS'es Skelegons.

Stop right there. Anyone doing what you're describing is already breaking not only the officially mandated rules, but also the basic rules of etiquette. You cannot argue about this sort of thing based on who can BREAK THE RULES BETTER.

EDIT:

Bishops train faster than every class in the game

Yeah, hi, this is because of available monsters, not because of an intrinsic value of Bishops. If Skele were weak to Fire and Ice instead of Holy, Mages would be hitting nearly 2x as hard as Bishops would be (assuming elemental weapon). Where do you think Bishops would be in this sort of world? A world that has plenty of Fire and Ice monsters but no Holy-- Bishops would be in a rut potentially even deeper than the one Mages are currently in.