> It could in principle be restricted to residents, or go away entirely.

If the UK loses control of it, I'd put most of my betting money on Option 3: The new owners extort everyone with a .io domain for a rate proportional to the perceived value. In other words, $50K a year for a successful tech company, $1000 a year for the average joe who doesn't want to lose control of a domain tied to 1,000 accounts.