I wouldn't say it's a myth. It may happen and may not. Please allow me to share with you some of my thoughts on the subject.
I can only offer you my very personal experience as well as my friend's whom I used to fist. I have been stretching my ass since my very early teens. In total, about 15 years. Two years ago, I sort of hit a wall. I experienced some problems and it turned out I had developed an internal prolapse that causes some very unpleasant symptoms. I won't go into detail, but it's basically something that quite significantly lowers my quality of life and there's very little I can do to aleviate the situation on my own. Surgery awaits me one day, sooner or later.
So being almost 30 years old, I am not incontinent. But throughout the years of anal enjoyement, I have developed other problems. What might follow is beyond me. Now, thats my story.
My friend is turning 60 next year. He's been stretching his ass probably for two or three decades. He can sometimes take two fists. Yet he doesn't have problems I do, nor is he incontinent. So what do you make of this? I don't know. Personally I think that sometime along the way, I would have ended up with the same condition that I have. I may have just speeded things up a little (or perhaps a little more than little?). Well or not. I may have just been too rough in my teens and spoiled myself for good. There's no way to tell.
Having said that, I can't really answer your question and I would be really hesitant to do so compared to some people on this board (no offense please). You say you want a prolapse. Maybe get some more facts before you get into it. Read up on it and just know that once you get it, there's hardly any way to undo it. Unless you're willing to do surgery.
Go slow, go wise and think before plugging in xD