I see your point. I've only known about three girls/women with hair reaching their knees or lower, and yeah, the ends often looked worn out and stringy. It reminds me of one long hair girl I used to follow online who had lots of followers and nearly had hair reaching her ankles at some point. She cut it eventually so it was slightly below her butt, and for me, it was a BIG improvement since it looked a lot healthier, even though it upset a lot of her fans.

My personal ideal would probably be nothing more than butt length. That definitely doesn't mean knee-length is a deal breaker, just that it's suboptimal to the former.