system
Given the following post, respond as if you're commenting on Reddit:
human
[Diablo] Could the angels have invaded the Hells as effectively as Diablo invaded the Heavens?

When Diablo invades the High Heavens, his attack actually goes well until the player intervenes. Cancerous, evil growths are erupting; you're attacked by "corrupted angel" enemies; he makes it to his ultimate goal and drains the strength out of even the archangels.

Of course, Diablo has a big advantage. He combines all the Evils within himself, while Heaven has neither Malthael nor Tyrael fighting to defend it. So it makes sense Diablo would win.

So what about the times prior to Diablo 2 when the Prime Evils were off the board and the archangels were still all present and united? Could they have attacked the Hells and won against the outnumbered Lesser Evils? Could they have made the pretty blue heaven trees erupt out of the ground and force demons to fight for them? Could they have fought their way down to some ultimate weak spot and destroyed it?
gpt
I'd say it's unlikely, even if the entire council had agreed to follow one voice and follow (most likely Imperius's) will for the entire campaign. The angels have always seen the eternal conflict as a defensive war, always pushing the tide of evil back into the pits it spawned from. They don't have the same motivation to fight that Diablo did once he was reunited as the prime evil. When all seven heads of the dragon Thathamet were as one, they were intent on ruining the last vestiges of Anu and wiping it from creation. Unfortunately, there's no way for Anu to resurrect in a similar way. At least, not without the extermination of the council and uniting their souls into a new soulstone. And that's not something anyone is willing to do. 

Plus, Diablo's new form held literally all evil in all the universe, a spear point that no force could hope to stop. Even when the Angiris council fought together, they were not as united, nor did they hold the collective force of good that Diablo did in the opposite way. 

It should be noted that angelic boots have trod on hell's domains countless times in the past. And there have been times it seemed like Heaven was on the verge of final victory, only to have the prime evils snatch it away at the last second. (that Diablo 3 animated film is a good example) But that's just how it goes when you're locked in a war called the Eternal Conflict.