No, it´s not all about quality, and waist length or longer does not automatically mean thin and stringy. This very forum is full of wonderful examples to show that it doesn´t.
This woman on the picture has incredibly beautiful and healthy hair indeed - which in my experience suggests that it would be just as full and healthy, too, if it were a foot longer.
If I knew her in person, I would encourage her to go for that length. And I´m sure it would still have that super quality then, and look even more impressive.
With hair as with almost everything else in real life, nothing is ever a one-factor thing. It´s always about a sound combination or mutual interdependence of quantity and quality.