OK, that's probably has been theorized again and again, but what do you think would have happened if the MIB had escaped ? Here's what I can picture : he would extend his powers to the whole world, « claiming » every dead, until everyone is turned into one of his mindless zombies, proving his point, that the whole humanity can be corrupted. Listen to what Widmore says ( and I think that, for once, he's telling the truth ) : he didn't say everyone would die, he said everyone « would cease to exist », which is a peculiar way of phrasing it. But if you think that the whole world would be populated by pre- « The Candidate » Sayids, that's an accurate description : the « claimed »'s personnality vanishes after a while, he ceases to exist. Of course, the « claimed » can still die and never come back … for instance, if they're split to smithereens like Sayid. So one might argue that there would be a way to escape the MIB's grip. But should the Light have remained extinguished ( can the MIB still « claim » if the Light is out, since his powers come from it ? I'm not sure, but let's assume so ) , if the Light is the same light that engulfs the losties in the church, the dead couldn't really … die. They'd be unable to « move on », probably stuck in some limbo, like Michael, or worse. ( When the Light was out, with that red glow, you would have thought you were in Mordor, on Mustafar … or in hell ) Fits with what we know, doesn't it ?