I have a hunch that previous generations wore boots for very practical reasons having nothing to do with ankle support.

Boots are very durable and can take far more abuse and wear than a pair of trail runners can. Older generations couldn't afford to replace their footwear constantly. I consider it likely they also did more off-trail walking than is usual today, because there were fewer maintained trails and more 'use' trails that were simply worn paths created by, you guessed it, boots.

Lightweight hikers of today are putting aside boots because they are responding to a new set of conditions, both in the woods and in the economy. If those conditions change again, boots might begin to make much more sense than shoes once more.