I didn't say it's realistic to move that much people around the globe, it did grow like that historically, but I do think, that migration because of this is becoming a sociodemographic and political problem in somewhere near the future. And is already somewhat in areas that are less wealthy and instable politically (e.g. northern Africa).

True, there's a lot of heating necessary to make life comfortable in Europe (but also in USA btw. probably even more, because I'd say the standards for insulation are better in Europe, and temperatures are more extreme more in the center of the continent).

But it's absolutely possible to live without (most of) the heating, by:

• Using passive solar energy (even in the winter) + good insulation
• Clothing! There are ways to help with heat too via evaporation chill though (I'm just sitting here with a cooling west, because I easily overheat, and that makes my life quite bit more comfortable in summer without AC, even or especially outside)

Though as you correctly notice, the combination of high temperature and humidity is what potentially creates a dangerous climate for life, even with things such as cooling wests in "low" high temperatures (within 30-40C), because evaporation chill stops working, so there are times and places, and these times and places will get more frequent where humans can't survive outside (without some serious technological counter-measures) while with cold temperatures you can always wear (somewhat specialized) clothing.

Evaporation chill does work even with quite high temperatures, but at some point (and I do think there are places that reach that point), the quite effective human cooling system is not able to catch up anymore (I think somewhere around 50+C IIRC).