So why couldn’t Smokie just leave?
So wait. Desmond pulled out the cork, and suddenly Jack and Flock can get hurt. That means they’re mortal. Which means smokie can no longer turn into the smoke monster, and so then is no longer “pure evil” and just another dude trying to get off the island.
So why couldn’t everyone just let him leave? He couldn’t do any more damage than anyone else, as his supernatural powers were no more. Because obviously, Jack had to pass on his “Jacob” role to Hurley, because he knew he’d die down there. The island protector can’t die from himself, which is basically what happened down there (Jack put the cork back in, lied in the pool being electrocuted, or whatever, and dying).
SO. this whole thing was for nothing, right? Cause all that was required was for smokie to become mortal again, and then everything be’d great.
Sigh. Lost, you are troubling to me.

I’m with you on this one. Sure we could assuming ‘Mother’ made it that way. It would have been nice if we had a firm answer in “Across the Sea”. Then, she told him he ‘couldn’t’ leave … which would have been fine if she didn’t have a ‘no, don’t’ attitude later (coupled w/the fact Jacob could come and go as he pleased).
That MIB wanted to leave (after destroying the island) was central to the journey of this group of people, and it is a little disappointing that a sentence or two wasn’t written to clarify it.