Umm some of us do that by necessity now. It really is surprising that the vast majority of members here really do live in a (quite comfortable and well off) bubble.
I guess before we start reacting to what's by necessity, we'd have to understand the living space. My main living space is about 1500 square feet. I have another 800sf of below ground partially heated area, and I do nothing to supplement what it is (it's cool in the summer and cool in the winter). When the mrs. is out, I shut about half of the house off and only use a single area A/C (this isn't England so you'll see temps here in the summer, and sun intensity that you may not see there - touching 100F sometimes, and perhaps a few weeks over 90F each year. That said, as a child, I lived without A/C and while I didn't like the heat, I don't remember minding it much. The summer I first worked here, I had no A/C and it resulted in soggy book pages and a leather coat (long time ago) that grew mold - some book pages did, too, from the humidity - but I didn't lose sleep (just used a fan, and certainly had to wash sheets more often).
I'd be willing to bet more than half of the households here and there generally don't have much in terms of uncomfortable spaces.
That said, if my parents talked about zoning the house, their first floor is the same as my total living space. They do zone their living space (no heat in the upstairs except for two three hour swaths at night and in the morning, which means temps in the middle of the night could be in the mid to low 50s F - they sleep through it and so did I.....but their "half house" heat and air bill is more than my total bill with the Mrs. here, sometimes close to double.
The floor plan of newer houses here is generally open except for perhaps an office and a couple of bedrooms. A typical one floor house will have three bedrooms, one separate small washroom area and then the living room, kitchen and dining room have decorative dividers, but only one room and the ceilings are high to make the area feel open. The bedrooms are about 500SF of around a 2000SF total - there's nearly no chance to zone anything.
I'm guessing the doors in older houses were there to facilitate heating only a couple of rooms. My parents' house is old (Despite being large) and in any doorways that there was no interest in having swinging doors, there were pocket doors, and the areas that didn't require a locking door had sprung "Swingers".
Of course, they removed all of that and the addition to the house is a single large room with a vaulted ceiling and radiant heating in the floor (and a separate A/C).....to get an "open and light extra living space".
