Does everyone so quickly forget the Light? And what happens when it goes out? MiB may not have had the intention to obliterate all humanity, but it would have been a side effect of getting off the island.
I think that's a stretch. We don't
know that for sure because Darlton chose not to tell us about the "mythology" of the Island -- that was on a need-to-know basis and apparently they felt we didn't need to know it. All we know is the nebulous dark "warning" that Crazy Mom passed on to Jacob, and it was pretty darned nebulous. And as nomteticus noted above, what we definitely saw as the short-term result of putting out the light was the destruction of the Island and making Smokey mortal again, and I'm having a hard time seeing that as a precursor to the obliteration of all humanity. (I suppose it could be -- maybe the island was just the first thing to go, and eventually
all the continents would've caved in, I dunno.)
As I've said elsewhere, I just wish they'd told us more about that mythology, so we would know whether the characters were right or wrong in their assumptions. Doesn't matter as to what happened, I guess, because people act on their beliefs, and if they believed as you do about the light then they'd act accordingly... I just wish I
knew, is all. The eternal human condition, right?