OK, so I've been giving this some thought, and here's what I've come up with.
Them leaving the island was never part of the original plan. I think that they came up with that story arc to help elongate the life of the show. Unfortunately, ABC gave them an end date for the show shortly after this all went down, forcing them to get back to the island to finish the overall story sooner than expected, hence how rushed last night's episode felt.
I feel pretty sure that the end of the show as planned from the beggining relied heavily on the people that originally crashed on the island as they were in season one, which would explain them having to replicate the circumstances of the original flight. I think that from here on out, we're going to see the storyline ad they've had it in their heads all along, starting with the crash. Only this time, obviously, they've crashed a second time under different circumstances.
There it is. Sorry for any spelling/grammar mistakes, I wrote this all from my phone.