If anything, we probably saw a glimps of what will happen when we all got the echo. (Dark skies, fire in the sky, etc.) Personally, I have doubts Dalamud will actually hit, due to a moon-sized object directly hitting a planet would wipe Eorzea off the map, and don't forget the mass-extinction events that would occur all over Hydealyn. (In that context, it makes the empire look like it's run by omnicidal maniacs.)

Rather, going by the early cutscenes from the main scenario quests, I think what will happen is that the beastmen will summon thier primals, who in turn will use thier god-like power to either redirect Dalamud, (like the lifestream in FFVII) or destroy it while it's still a distance away. However, regardless of which scenario happens, it will be too little, too late to save the land from an indirect hit, Hence the massive star shower from the early cutscenes, the screwed up aetherite camps, etc. We'd still see a major world-shattering cataclysm, but still have a believable means of surviving to tell the tale.

Just speculation on my part, feel free to add your opinions.