both timelines exist by Mr. Awesome

Try this on for size.

Its Schrödinger's cat paradox with the black box, the cat, the vile of radioactive juju beans, and the problem that the cat closed into the box with the vile is simultaneous both dead and alive.

Both timelines exist. Because each cannot exist without the other.

If the whole crew (newbs) does not get on the plane in Australia, they can't have crashed on the island. And if the (crashies) team doesn't detonate the bomb, the plane cannot fly past the point where it crashes.

Meaning that neither the newbs nor the crashies can exist without the other.

Now certain problems persist, like what to do about the hatch being built and eventually blown to bits when desmond turns the key. Which the "crashies" at the swan discover still took place, when they come to 30 years after they've detonated the bomb.

But those events had to and did in fact occur to the crashies. They could not be standing where they are looking down on the blown bits swan had they not found the hatch, and had desmond not turned the key. Despite the fact that they are jumping back and forth in time - their running individual clocks cannot be unwound. Their aging is not reversed. Their personal history cannot be erased. If time is a ribbon from beginning to end - they are individual threads that have become for some portion detached and in that portion got all knotted up around the primary ribbon - jumping back and forth along its length. But their individual strands have a beginning and end same as the primary ribbon. The portion of their strand that has happened to date cannot be reversed, even if they do something that would otherwise change the course of the larger primary ribbon.

(likewise because the hatch is blown up in 1977, and then not rebuilt for the newbs who actually existed in 1977 as kids, there is no button for desmond to push - and hence he is on the plane with the newbs.)

And so you have Juliet on the cusp of death, in fact crossing the threshold, where the universe aligns and allows a moment of collapsing clarity, and her message to sawyer is, "It worked."

Now if we are to believe that the two time lines are infact occurring simulaneously - albeit in sortof alternate dimensions now - we must also realize the parellels that would otherwise be occuring between the two dimensions would occur no matter what.

I know the writers are spoonfeeding them to us, and i'm sure alot of theories touch on them. But just to refresh a few here:

-John and Jack discuss the appearance/disappearance of christians body on the beach and in the lost luggage claim area

-Charlie nearly dies on the plane from the drugs in both scenarios.

-Kates marshall recieves the same injury to the head, one because of flying luggage and the other due to bathroom counter.

-Locke and Boone, have their endearing but brief relationship.

-Ethan dealing with Claire and her pregnancy.

-etc.

Needless to say, if the crashies version of Juliet dies in one dimension, she likely does so in other as well.

The suggestion of these events that occur to both dimensions is that a certain trajectory is inevitable. Fate. Ultimately, the line is going to end up in the similar place regardless of whether they crashed or didn't crash. The infinite end of the primary time ribbon "only ends once."

The opposite suggestion, "they come, they destroy, they corrupt..." is ofcourse the inclusion of free will. The same free will that apparently does effect the trajectory of the ribbon - but not without denying those previously stated fate "certain" events which transpire along the way.

And so we end up with the same theme that's been perpetuated throughout the entire series in some measure or another. Freewill vs fate. Locke and Jack. MIB and Jacob...

And the perpetual question of is it freewill, or fate?

Alas, much like Schrödinger's cat is both dead and alive, and both of the two timelines exist; the driving impetus for the trajectory of time is both, freewill and fate.