Your question is commendable (if sincere). In extreme hot weather, in the high eighties or nineties there is a danger of dehydration. Personally I get very severe headaches easily and to be able to function (go to work, take care of my family) I need air conditioning. Most people will need it at even much much lower temperatures than that to function properly.
I think most people would feel they can do mitzvos better when feeling well – daven, learn, etc. Also, children need a geshake, happy house. They shouldn’t feel they’re suffering. (See Rav Brezak’s articles in the Yated on having delicious food and treats on Shabbos) . I imagine the Steipler didn’t eat cake or ice cream. If a person wants to limit their gasmius there’s probably many areas to start with before removing air conditioning. But for himself, not his children or spouse. Maybe limit noshing to only on Shabbos and other things like that.