Back to the Future..
As anyone who’s familiar with the “Back to the Future” trilogy knows, in order to change the present/future, you must alter variables before they impact reality. Going forward in time AFTER an event has occurred means that you’re going to end up in a time/reality that has recorded that event as history. Other variables may have changed as well, resulting in, perhaps, a somewhat altered reality, but the logic of time travel holds that whatever time you go forward to, any event that preceded your movement through time will be part of the history of whatever reality you end up in.
As Juliet hits the nuke with a rock, it apparently detonates. When we see the Season 6 opener, the island is somehow submerged into the ocean, giving the impression that (given it’s direct connection to our “new” alternate time-line where 815 does not crash) it was “sunk” due to the nuke. However, when we see the 70’s Losties crew arrive back in the future, they are walking around on the same island/in the same reality as Flocke, Richard, Ben, and Sun are. If the nuke causes the island to somehow “sink”, the 70’s Losties would have time-traveled to the island in that state, since their movement through time occurred AFTER the nuke explodes, albeit only being perhaps a nanosecond. They should have transported through time and been immediately underwater.
This theory is, I believe, only negated if, A) Cuse and Lindelof are using a different logic of time travel or B) the underwater island is somehow NOT the result of the nuke.
I think they are using the idea of two timelines, one where the bomb doesn’t go off and the island doesn’t sink, that’s where the 70s losties are as that’s their native timeline and a new timeline where events have been changed due to the bomb. Both timelines are running seperately yet side by side. They will write them back together somehow.
You didn’t really posit a theory, you just compared lost to something else. Clearly, the losties did not jump to their relative present and start swimming; they cycled back to realize that it’s all the same. Remember “whatever happened, happened”? They can’t change anything, because they ALWAYS exist in the same reality in which they started, and no matter how hard they try, it will all be in vain. I think the point of the alternate timeline is going to be a little lesson for the audience in making good choices.