The island time line is fabricated - they will see each other in another life by SurleyGirl

I have always argued that the original time line was Alt all along. Well how about this. The original time line has been manufactured by the island as part of this "game" Jacob and MIB have been playing all along. Jacob flips key circumstances in critical characters' lives to set them up for a path that is "bad" in relation to what they originally experienced (Sawyer is a con man now instead of a cop, Kate is actually guilty, Jack doesn't have a son to find redemption in, etc.). He brings them to the island to prove MIB is wrong because despite this bad path he has set up for them, they are still capable of good, and this is demonstrated through their struggle on the island, and by key characters returning to the island after getting off. The jig is up though because Desmond now knows about the other time line. His mission is to merge the two by triggering memories of the island in sideways world because he knows they are all going to die on the island - by allowing everyone to rediscover the love they found on the island time line, they aren't losing the value of what they gained in the original time line. When all their memories are merged, the game is over and the island loses. Jack is not going to replace Jacob and keep up the island's shenanigans, he is going to do something to end the island altogether, and it will be alright because, as Desmond is fond of saying, they will all see each other in another life.

I also think that what Desmond is doing is not what Widmore intended for him to do. I think Widmore's motives are strictly selfish and he wants to use Desmond for some other purpose that involves electromagnetic energy (whether it is to destroy MIB or the island itself, I don't know). When Eloise, the oracle like figure who has knowledge of both time lines (and who may have assisted with fabricating the one), sees that Desmond is keen on meeting Penny, she realizes he is doing what he did before (jumping around with his consciousness) and that he is deviating from the plan, which is why she tries to discourage him. I am sure the last thing she wants is for Daniel to remember this other life where his own mother shot him and then pushed him down the very path as an adult where this was bound to happen. Desmond, the one who is capable of acting like a variable like Jacob did in the island time line, is bringing the best of this other world to the Losties so they are completely fulfilled.