I don't consider it a huge stretch--it's just that if ONE event never occurred, the entire time-line of events that happens after it would never have happened. Breaking one link on the chain causes the rest to fall to pieces--everything leading up to this point is caused by issues that are all linked back to the Well of Eternity.

If the elves hadn't meddled with the magic of the Well of Eternity, they wouldn't have drawn the attention of the Legion and gotten corrupted. We wouldn't have the Legion knowing about Azeroth at all. Without that corruption, we wouldn't have The Sundering. And without the Sundering the Legion would not have known of the Orcs and looked for them. And without the Legion's involvement with the orcs, the orcs would have stayed and lived their primal, care-free shamanistic lives in peace on draenor with the draenei. Without the Legion messing with Gul'dan and Ner'zhul, there would be no Lich King. And, yes, without the Lich King trapped in the armor in Northrend, Arthas would not have gone corrupt--there would have been nothing to corrupt him. The creation of the Scourge would never have occurred.

With the Night Elves didn't directly create the Scourge, its creation would have been made impossible if the Night Elves never so recklessly messed with magic beyond their kin. So much pain and suffering could have been avoided. While the elves are not directly responsible, everything that has happened is directly tied to the Well, which is what makes the War of the Ancients the single most important event in Warcraft history, and this is why we see the elves around everywhere.