That's a severe case! But as you say, showroom demo model, sits in the shop for months with the screen permanently on at full brightness and mostly showing the same demo screen - it could scarcely be better designed to produce burn-in, but it's nothing like most people's real usage.
The closest to that I can think of that might happen in real life would be someone who drives for many hours every day using the phone for navigation in bright daylight (so high screen brightness). The map itself would move, so would not be a high risk, but parts of the display that always showed the same object would be vulnerable. So if you do that for a living and insist on using the phone that way then maybe I'd be wary of AMOLED - though if that is your life you could just buy a satnav and then you don't have to worry. Otherwise though it isn't a big concern in my experience.