Spongebob is basically like the new Looney Tunes. It will endure for a long time.
I don't think either South Park or Family Guy will be relevant within a few years after they end. Family Guy's audience is just the right age for 80s nostalgia, but younger people aren't going to get it. It'll feel like 70s nostalgia does to a child of the 80s, all the dated stupidness but none of the warm memories that give you that fuzzy feeling.
And South Park, as Darth Gonzo pointed out, does shows about things that are currently happening in the news, they can turn an episode around in days. That's just a little too timely to be remembered well. Some of them are already dated, there's ones about presidential elections and people who the public hasn't really thought about in years, like that one about that guy who used to have a show where he claimed he talked to dead people. I can't even remember his name, that's how much he's fall out of popularity.