Thus said, there are plenty of places around the Mediterranean which have always been pretty warm in the summer, and where there is a convention of closing down between noon and 4 PM, but spreading the work-day longer during the evening and the early morning (think 8-12 4-8 instead of 9-5). So I strongly suspect that there is much about this that is a matter of culture and habit.
(Not to mention that your use of modern is pretty wrong historically, since AC is a contemporary invention, definitely not modern, but I'm digressing...)
Works in many hot places around the world. AC can be complementary, but you dramatize.
Most of our electric bill in the summer goes to the AC unit(s). Some people have evap/swap coolers but those only go so far, and stop working during monsoons cause it's already too humid.
Edit: its 8:45am and 97f outside.
Modern society would not exist without air conditioning, it is completely necessary to sustain our current civilization.
I'll concede that adding air conditioning capacity will increase global warming, but the above point still stands. Without A/C, our civilization is not sustainable.