Yes, it can happen! But it was a very disjointed process. There were periods of time when I worked on it, then I would put it away and literally not even think about it for years. But at least I always knew it was possible to finish it, as I had worked out the major plot points when I first started. It was just hard to keep going on it because of its length, because I wasn't sure if anyone would ever actually want to read it (small fandom), and because I became distracted by writing other fanfic with a more immediate reward.

There's another type of WIP, the "I really don't know what happens next" type, that is a different challenge to finish. I have one of those, too, but I'm now inspired to go back and try tackling it.