Either ending would seem to have to have the Losties continue their life from the past, before flight 815 took off, (2004?) and the past 5 years would simply "vanish". Not sure I like or agree with that idea, but it could happen. They know what happenned in the original 2004-2009, so it would also require them to be in a weird state of the past for the rest of their lives, unless the memories were somehow erased from their brains.
It would require an alternate timeline from 2004, like a Back to the Future type of thing and then we get into the never ending dispute about alternate timeline or the past has happenned and it can't be changed. (known as what happened happened or WHH)
I beleive teh writers/producers have said teh show is more about the characters, So, if true, I think all that has happened to them from the crash has happenned to change their characters - for example Kate, you burned her step dad to death in his house, has shown incredible passion for young Ben, even knowing the hell he will put her through when he gets older. So, to erase all that, by going back to 2004, would seem counter productive.
Right now I don't have a guess as to whether they live out their lives on the island or get off though.