I have a few opinions I'd like to share on drop-outs. Please let me know if you think I am way out of line.
I think the first reason someone will drop out is cultural. We are in a "want it now" quick results society. Aikido doesn't give you that. For many, it may take years before you start getting what I like to call "fleeting flashes of insight". (Those days in the dojo that you just "get" a technique and everything works perfectly) Of course then the next time you train the technique it doesn't work
On this line, it will also depend on what the person's goals are. Aikido is not overtly conducive to short-term goals. A person may come in with the goal of reaching a black belt. (Will they fail because its too hard, or will they succeed and move on after they attain their goal?) Depends on the person.
This leads to the second reason people drop out; frustration.
Frustration over technique not working. Frustration over the instructor giving the same advice over and over and not being able to absorb it. (The definition of insanity is: Doing the same thing over and over and expecting a different result). Frustration because ukemi hurts at first. Frustration because many techniques are painful. Frustration over things outside the dojo.
This leads to the final drop out reason. There is something more important, or the "I just don't have time" excuse.
The funny thing is I don't think Martial Arts are unique in this respect. I would bet that kids that start tap dance, ballet, or lessons with musical instruments have just as high of a dropout rate.
But, the good thing is that Aikido has an almost Darwinian effect of weeding out the weak (not the best term). So, for the most part, Aikido's senior people should be stocked with fairly high character individuals.
Just my thoughts.