Alt timeline has no future because Desmond doesn't turn the key by jesse

So, say 815 doesn't crash and creates an alt timeline. Also say for whatever reason, Desmond still winds up on the island. At the point in time that he had decided to off himself, but was saved by Locke pounding his fists on the hatch door, there will be no Locke pounding on that door. If Desmond does indeed kill himself, and the button doesn't get pushed nor the failsafe key get turned, then any alt universe ends. If that were the case, then the only universe that survives is the one where 815 crashes.

Even if Desmond doesn't make it to the island, at some point, if whoever is pushing the button doesn't get relieved and dies or decides to see what happens, then the world assplodes.

If this is correct (longshot), we could wind up watching this alt reality until the sky goes purple and it abruptly ends, and then again we'll only have the story or timeline we've been following. It will also give resolution to Mrs. Hawking's comment to Desmond that if he doesn't do these things, "every single one of us is dead.".

Apologies if someone else already said this.