Pet theory: most Dvorak users were, in their pre-enlightenment lives, messy freestyle 3-finger typists.

Given that Dvorak tries to maximize alternating hands when typing consecutive characters [1], that theory definitely feels plausible given that the "hunt-and-peck" style for typing naturally seems to work with alternating hands. I think the same idea could also be applied to mobile typing as you only have two thumbs — perhaps Dvorak would lend itself well to mobile typing?

If you ever went to the trouble of formally learning to touch-type Qwerty, moving to another layout just seems impossibly foreboding.

It's not that bad. By my experience, having gone from QWERTY to Dvorak to Colemak to Workman, it takes maybe an hour to memorize the keys, then it's just a matter of practicing by using it. You will progressively get faster and faster as it becomes second nature. To get to full typing speed and for it to feel completely natural, however, it will likely take a month, depending on how often and how much one types.

Something interesting that I noticed, though, is that it seems that the brain is only to be able to know one keyboard layout well at a time. If I learn a new layout, I don't maintain my skill with the previous layout minus the skill lost due to lack of practice. It almost feels entirely zero-sum. As I gain skill in one keyboard layout, I seem to equally lose skill in the previously known keyboard layout. I do try and maintain some level of proficiency with QWERTY, given that it is still the standard and is the most common, but it takes considerably more effort. It seems to be less acquiring a new skill and more rewiring the brain.