we pessimistically assume that learning a new layout will magically reduce your old layout skills to hunt-and-peck

That basically happened to me. I've switched away from QWERTY in 2006 and really only ever use it if I fix something on my mother's computer. Previously I was typing at 80-90wpm, now I don't even know where the keys are anymore. I could maybe get back to 30wpm with a week of practice, but most of my QWERTY skill is gone.