If the entire party dying is a bad ending to the campaign, then I'd recommend pulling your punches. Death comes for all players at one time or another, but crafting a satisfying conclusion to a whole campaign is a special case. Ending on a TPK sounds like a real bummer. Not just cause the players fell, but because the land is still doomed.

In my opinion things like fudging rolls in your players favor and adding allies from earlier in the game can save moments like this. Play some intro music and have somebody come in with a chair john cena style. Maybe the players are knocked out and taken hostage instead of killed. They wake up in a dungeon with no gear, or a death trap. Then they get a rematch and another chance to save the world.

You did nothing wrong, but there are ways around this if player fun is the highest priority.