Everytime you walk in or out the front door you get air exchange and heat loss. There’s no such thing as “NONE”. Not to mention exhaust fans in bathrooms and range hoods.
As for your theoretical example, the best way to conserve heating costs is to make people live in huge apartment buildings. These building typically are running the A/C until christmas. The larger the building, the lower the surface area to volume ratio. And if they are covered in glass, the solar gain is considerable.
True, fat people don’t need heavy coats, volume to surface ratio. But tell that to deer, bears living outside thru the winter. They wouldn’t survive if not for those insulating fur coats. Massive apt buildings have their own problems, most people like to live in their own home on their own land, the castle/turf instinct.
Thus exterior window shutters : operable, insulating, automatic-on-photocells at a decent price would save oodles of heat=money pouring out of those windows during the long winter nights. Then too, in hot climates they could close on the south side to prevent too much solar gain(less AC required). They might also be solar cell powered as you only need the motor for a few seconds twice a day. And too, there are batteries being developed now that are as thin as paper.
Maybe then some insulation manufacturer out there is listening...