It was unfinished for a very long time; I don't think it changed much between the 50s and 70s. Since then, though, they've really got a move on, and it's much nearer completion than it was when I first saw it about 1980.
But no, I don't think people go to see it simply because it's unfinished (though it is rare to see a slow, long-term project like this being worked on). Gaudi's architecture is a major draw for Barcelona, but I don't think it matters that almost all of it, except this church, is finished. What might matter is that much of the newer stuff doesn't actually look very Gaudi-ish.