The site is way more than just Minecraft at this point, if you look at wall posts you'd realise that most of them have nothing to do with the game. The friendships made is what keeps them here, but like any social media site, some get bored and leave eventually.
PMC as a community will thrive like it always has. Sure, there have been a few notable individuals who have left, but that has always happened and they will be replaced by new users eventually.