My 2pence worth
Forgive an inexperienced view (and a rather long one...)
I took up Aikido very recently, so I'm not really qualified to judge what makes people drop out over the long term - ask me again in ten years!
The search that brought me here was a long one: I'm thirty five this year, and it's taken me until now to learn the beginnings of patience. A vague interest took me to karate in my teens but I lasted no time at all as I had no patience. Work and life have intervened since, as have a few injuries, including one to my back (lifting! just say no...) which can put me down for a week at a time if I'm daft.
I reached a point where I realised that how good my life is could one day be dictated by how my back feels. I have exercises which are meant to help prevent this, but I work long irregular shifts, and I was losing the motivation to do them.
It took me two years to find aikido as a way of answering this problem. Actually I found Aikido in six months - it took the rest of that time to pluck up the courage to make the phone call. Thus far I have no problems. I have no intention of dropping out since I feel like I have found a home in Aikido: it seems to suit me physically and mentally. Obviously there will be techniques that I will have difficulty with, but so far so good.
I feel that drop out rate may be directly related to goal/expectation. I see Aikido as a long term prospect, to improve my balance, posture, timing (and judgement?), thereby reducing a number of problems I may have as I get older. I am not attempting to 'be a black belt by age X.' Other people's goals are different. Incidentally, why is it that non-practitioners only ever seem to ask 'how long will it take...?'
Perhaps because I find this important for my future I may be less inclined to quit, but I suspect that nobody intends to drop out; it just happens.