There's no logic in the ending either, really. Was there any point in the the season 5 explosion? It seemed to create the alternate timeline because the plane landed but then we find out that was just a red herring. It was actually a purgatory of sorts which coincided perfectly with Jack dying on the island....hmmm.
There was a point to the explosion. It brought them back to 2007 time instead of 1977.
It seems that a lot of people assumed that the H-bomb explosion created the alternate timeline in the flash sideways, and now they are unhappy that they were "fooled."
I have three problems with this:
1) It's absurd, even in the context of a show involving time travel. Why would detonating that H-bomb have done what Farraday and Jack hoped it would do? Is there any scientific theory that supports that? No. As Farraday said, "what happened, happened." THAT was their present, so how would obliterating their present undo things form their past? It's nonsense, and there was never any reason to expect otherwise.
2) The producers had said all along that "there was only one timeline" and that they wouldn't burn the viewers by having some sort of ending such as "The island was never real" or "They all died in the original plane crash" or "The Island is all someone's dream" or something like that. Which means the ISland timeline was always the one and only real timeline, which means it should not have been a surprise that the flash sideways was not real.
3) If the H-bomb HAD done what Farraday wanted and had prevented the plane crash in the first place, and the flash sideway was now the reality and the plane arrived safely in LAX, then why did the Island timeline continue? It should cease to exist, at least for the plane crash survivors, since THE PLANE NEVER CRASHED ON THE ISLAND!