I understand what you mean, Bart, having hyper-extended knees myself. You've been right with everything you've said about them so far and the challenges they pose to dancers. Many dancers are hyper extended, giving them a curvier standing line in arabesque. It's in "fashion" now but definitely was not in the past.
I also understand. My hyper extension is actually in the elbows and i fought against the backward bent look you describe my entire career and still to this day in tech. class. It's been a lifelong war for proper "line" and the hyper-extension wins 75% of the time, unfortunately.