The length and scope of a RP don't have to be mutually exclusive. For example, I once participated in a RP the premise of which was that the world was ending in some cataclysmic event and it was ending in exactly 21 days from the start of the RP. Each post covered the events of one day, so the idea was that we had to wrap up the whole thing in 21 posts. People played monarchs, generals, religious leaders, heroes, villains etc - so the scope was pretty big, but there was a definite end, both on an OOC and an IC level. Funnily, we got until about day 16 before the RP died out, but yeah, you get the idea...

Ultimately, it all depends on the people participating in a given RP. I'd go as far as saying that scope/length/complexity don't really matter when it comes to the longevity of a RP. I've taken part in RP's that have lasted for years and ones that have died out a month after being set up. It's a mix of a GM's willingness to see it through and motivate their players, coupled with said players taking an active part and supporting the RP to the best of their abilities. That doesn't always happen because we're amateurs and writing isn't our top priority - RL happens, people get burned out or lose their interest, etc. 

If it makes you feel better though, consider that a lot of professional, published writers fall into the same trap. Like, let's take George Martin and Robert Jordan, two very famous fantasy writers, for example. Have you noticed how concise and on point their first books were, while the later ones get more and more drawn out and tangled in their own complexity? Yeah. In fact, it's almost like a recipe for most fiction writers at this point: book 1 - 400 pages, book 2 - 700 pages, book 3- 1000 pages and so on. The more invested you get in your writing, the more you want to expand on it, which just makes the whole thing spiral out of control.