In what way is failing this criterion a failure? Where does it prove that it is necessary to satisfy this criterion?

Yes, this is a great yoga position, but my yoga teacher always says to be aware of pain at the top of the thigh in the back. That's where your various hamstrings attach to the pelvis, and to injure the tendons there is a long recovery. We're taught to stretch as far as possible but do not cause pain at the ends of a muscle. You want to stress the muscle, not the tendon.

After many years of doing forward (back, glute, and ham) stretches and never getting much relief from my lower back pain, I discovered McKenzie: https://www.spineone.com/blog/mckenz...hod-back-pain/
Fixed me right up the first day I did them. A revelation.

To answer the original question, I'm flexible enough for everyday life. Never saw the point of trying to increase flexibility, even before the studies started coming out that reported damaged connective tissue in some frequent stretchers.

I understand that stretching can be essential for people who have to live with certain kinds of chronic pain. That doesn't describe all the people posting about their stretching regimens, though.

At 67, aside from arthritis in my left elbow from breaking it at age 5 and again at age 11, I have no joint flexibility problems. I can probably thank having been a lifelong cyclist for that. Sometimes it sounds as if talk of how much people can bend is simply a matter of bragging rights. What is flexibility beyond the norm good for?

I started stretching at about your age when I started to get bursitis in my knees from my tendons being too tight. No more bursitis, and I only had to pedal about 10 miles without using my left leg and just that once before I fixed it. Before that, ordinary strength work in the gym had been enough to keep using a slammed -17° stem on my racing frame. Yeah, you're good now and maybe forever but age happens.