Ah, come on. You asked for it, didn't you? You wrote "I'm guessing it's a special magic planet with startlingly good weather year-round, without disabled people, where everyone gets to age into spry Euro-oldsters". Of course, such an extreme statement will trigger a reaction from people who actually experience every day what you claim to be impossible. It's not possible in every city and it's not possible for everybody, but it's possible in more places than we think once we accept the idea. One quarter of the Dutch population cycles every day and the number of cycling people (obviously lower than the average) over 65 is actually increasing thanks to better a infrastructure and, of course, e-bikes.
And talking about hot climates: You remember the pictures from South China in the 1970s or 1980s where everybody was cycling because cars were too expensive and public transport was bad? Now everybody has a car and nobody with a sane mind would cycle in China.