Losties Need More Info

So we know by now that many people were once candidates, but somehow ended up being crossed off by MIB in the cave and by Jacob in the Lighthouse. It’s made me wonder if the Losties could somehow un-candidate themselves on purpose…? And if they could, would it be a good thing or a bad thing?

MIB said he needed all the candidates to leave, but really he wanted/needed them all to die. If they were all dead he could leave… Is is the same if there are no candidates left? What happens when there is no one left to protect the Island? Does it sink?

In that case, does everyone get to go on living in the Alt world?