I'm not sure those people are really "letting themselves go," I don't know that they ever had a hold of themselves...

A lot of it I have to blame on the medical establishment, honestly. It seems like every time I've had an injury, from age 16 to relatively recently, the only advice I've ever gotten from doctors is rest, rest, rest. I had to figure it out on my own that after the acute phase was over, resting was making it worse, and that in a lot of cases, if I stopped resting (and maybe made some tweaks in form or strength), the problem would go away entirely.

There are a very, very many people out there who don't exert themselves because it hurts. Many of them have been my clients. I would never advise someone to ignore pain, and I really try not to discount others' pain. Unfortunately, these people have just gotten very bad advice when it comes to dealing with their pain.