Don't misunderstand me; I'm not suggesting that every single role-player you'll ever encounter lacks the devotion to stick through 'til the end, but the vast majority of them likely will. This is why I mentioned using the OOC, and a general gauge of the participants' personality, as a good way to work out if they should be accepted into a role-play.
You could make the argument that, as a writer, I am in fact role-player with myself since I get into the minds of each character when write their part of the narrative... That being said; I don't think you should completely give up on finding people who are worth investing what you have in mind into, since they do exist and are simply difficult to find. So long as they're not inherently a whimsical person and life doesn't get in the way you should be fine so long as they were genuinely interested in the idea from the beginning.
The problem with me is that I just don't have faith in people. The sheer amount of bullshit a single person is capable of makes me leery of other people from the start; and in my roleplay experience I've only found a dozen roleplayers I can truly say stuck to... I can't say the end, the group never made it to completion, but it did last a long time. The shorter roleplays I've done that actually have been completed are ranked higher than those that I've had great aspirations for. The feeling I get for completing a roleplay, however small, is much more worth my time investment than developing a world where I can't get a single story done in.
Will I completely tell humanity to fuck off? Well, I don't think caves give me brownies, and since I like my semi-daily brownies I can't say that much, but I do reserve any degree of real trust for those who earn it over an extended period of time. It is those people that I start to have a little trust in for getting things done.