Usually most rps die out as people are no longer able to post, and others loose interest or things get stuck, and no one wants to replace them and so on, and also if there are not many people online there is a smaller pool of people to choose from. But that's the only way they will end unless who creates the rp has a clear end in mind, and or rules to get to that end.
When I get my rps ready to go I'll be bribing people to achieve the goals lol!