Juliet is the key by Lapidus's_chesthair

I still refuse to accept the sideways as just purgatory or some spiritual limbo and believe what we were seeing in season 6 WAS an alternate timeline created by the detonation of the bomb.

But of course if we're to accept this then what was the original timeline? Mrs Hawkings conversations with Desmond in 'flashes before our eyes' and Widmore in 'the constant' seem to confirm that she had always shot her son and there was nothing she could do to change it - "don't talk to me about sacrifice charles!"

Also Sayid & co infiltrating dharma could lead to the war mentioned by Mhikail and Ben when talking about the purge. What i accept is that in the orinial timeline or loop if you wanna look at it revolves around the time-travelling losties trying to avoid the incident but ending up creating the circumstances which led to them coming to the island and travelling back in time in the first place, thus the paradox of time travel and a continous loop which cant be broken.

What was different i believe is that in the original timeline Juliet had left the island, either by sub in 74' or forced off with Sawyer in 77'. However this time she made a choice to stay and ended up in the hole where dharma were drilling and was the one who set the bomb off and rendered the energy inert so that the button/hatch would not need to be built (just like faradays plan). This created the alternate timeline which runs parallel to the original timeline with subtle differences. Notice that the most major difference is that Juliet has given birth in the alt timeline, in an ironic twist the woman who was brought to the island to solve the fertility issues ends up creating an alternate timeline and gives birth to a new life.

At the start of season 6 we see the island is underwater and effectively destroyed. Remember Jacob and MIB's mother telling them that if the light goes out on the island it goes out everywhere. This is what is happening in the flash-sideways, because Jack and Desmond never came to the island in this reality they were never there to save it. Jack and Desmonds destinies were always to save the island/destroy the monster, it's been drilled in by Mrs Hawking, Locke and others all throughout the series, because of their absence the island sinks either by MIB trying to escape or by other means. The scenes at the end where people remember the original timeline and 'move on' represent this reality coming to an end because the island has been destroyed. This is what Widmore & co refer to when everything would cease to exist if the island wasn't saved.