No problem. This is probably one of the hardest things about the show. Not that it's hard to understand, but that it's hard to accept.

They don't explain why it'd be so bad. We're stuck imagining what the reason could be, and we have practically nothing to go on.

We're led to believe that everyone everywhere would die. That's all we've got.

Here's the best I can do:

We have to work backwards.

It's important that people not find the island. Why? Because they'd find "The Source" (the light). Why is that bad? Because then they'd try to take it, and in doing so, the light would go out (i.e. the Source would lose its power).

Why is that bad? Well, that's the billion dollar question. And the answer depends upon what the source is.

My personal theory is that it's the essence of life. I'm not sure I like that explanation too much, but it fits what they've done with the story. Men want more life, and they'd try to take it, and in doing so, it would deplete the pool of life essence. Perhaps that would mean no new souls would be born. Perhaps it would literally mean that life everywhere would be extinguished. I don't know.

So... If the MiB leaves, we have two potential problems. First, he may bring people back to the island to harness the Source (something he had been trying to do before, albeit to leave). Second, given his smoke form, he's an incredibly powerful creature. He's invincible, he's able to read a person's memories, and he's able (and apparently willing) to kill whomever he wants.

Is MiB evil (or insane) enough that he'd kill everyone on Earth? I don't think so.

So I'm extrapolating that it would mean the island would be left unprotected (Jacob has to be dead for MiB to leave), and people would come to the island to get the Source, whether by MiB's influence or not.

I agree with you, though. I LOVE the show, and I loved the finale, but I'm disappointed with how they were to outrageously vague about this particular thing.