And in the "Charge of the Aspects" short story, Kalecgos says:

"But this is just the beginning of our tests," Kalec quickly added. "I believe we must have a tool at our side when we face Deathwing. Numbers, no matter how great, are of little help. We require a weapon... like none that has come before it. My flight will not rest until it solves this predicament."

Now, to be honest, I don't think is makes a lot of sense that "numbers, no matter how great, are of little help". But the blue aspect thought so, and the other aspects present don't disagree with him.

So it might very well be true that even if all the good guys of Azeroth had teamed up back at the start of Cataclysm they still couldn't have killed Deathwing (until he's weakened by attacks with a charged up Dragon Soul, of course).

@Methuus

I'll quote Thrall & Alex on this from "Charge of the Aspects":

-Thrall: "I do not mean to question your wisdom," Thrall said humbly, "but other races throughout Azeroth have also suffered Deathwing's fury. We could muster an army of mortals the like of which has never been seen to crush the black Aspect. Would that not be a simpler
course of action?"

-Alex: "Even if every living mortal faced Deathwing, it would matter not," Alexstrasza said. "He has been twisted by the Old Gods' dark energies. No physical assault, however immense, can destroy him. He must be... unmade. His very essence must be unraveled, and the Dragon Soul alone has the power to do so."

Although Anne clarified why we didn't go straight after the Destroyer just after the Shattering, you can see here that it would be futile.

Yeah, I know what all the NPCs are saying. I just think that story driven invulnerability is weak story telling. So I'm not saying the NPCs are mistaken, I'm just calling out Blizzard on a lame plot device.