Agreed with the initial 2.5 paragraphs of your post. But
if you want to strengthen a muscle, you have to exercise it, and in general the more intense the exercise, the greater the gains ............ in general exercise leads to better health. By logical inference, better health would obviously lead to the likelihood of living longer
Exercising a muscle strengthens it can be accepted. But I don't see any increasing function between muscle strength and health. And it is another leap, though a shorter one, between better health and living longer.
Very weak people are unlikely to be healthy - but after a certain threshold increasingly more muscle strength definitely doesn't lead to better and better health. This threshold isn't even hunk level strength. Note also that health is typically defined in a negative - absence of physical and mental disease. Not only does this correlation NEED a scientific study, there weren't any good ones that I could find last when I looked a few years ago.
And people very healthy while living drop dead suddenly, or after a very short "non-health". And non-healthy people live on 20 pills a day for 40 years. Both these effects run into families. This is another correlation that isn't as much as it is generally assumed.