Personally, I would use the iPhone 3GS as the baseline minimal hardware that can support all of the must-have features of real-world software, because the 3GS was the first model capable of running the VoiceOver screen reader (in addition, of course, to all the other things it could do). But then, I'm sure I'm over-emphasizing the must-have feature that matters to me.
I think both comments are true. Take something as lean as GEOS and upgrade it to support 5K displays, HDR color, unicode, etc., and I still think it would be several orders of magnitude smaller and faster than most of today's software.
Personally, I would use the iPhone 3GS as the baseline minimal hardware that can support all of the must-have features of real-world software, because the 3GS was the first model capable of running the VoiceOver screen reader (in addition, of course, to all the other things it could do). But then, I'm sure I'm over-emphasizing the must-have feature that matters to me.