There is a ubiquitous phenomenon in Germany, called "catching a draft". It's a real thing which as far as I can tell only befalls Germans, with physical symptoms and everything. On top of that and somewhat paradoxically they do believe you
have to open the window every hour or so, even if the building is sufficiently ventilated in theory, otherwise they get headaches, complain of unbearable CO2 levels and/or a stinky atmosphere. For something to be unhealthy, it is sufficient to
believe that it's unhealthy.
Even though I was born in Germany, I never really understood these compulsions, but they do seem to run very deep (I'd say 50% of my German friends exhibit them on a level that can become disruptive).
I suspect in theory the temperature variation experienced by going for a swim in a lake during summer (which is an activity generally enjoyed by Germans) should be larger than what you get from moving in and out of a properly air conditioned building. It probably depends hugely on individual lifestyle, but I think ubiquitous air conditioning would in fact reduce the temperature variations a person experiences during the day. Even in a country like Germany, people do tend to spend most of their time indoors or in their cars.