Sorry to challenge your inept logical fallacy wankery, but a lack of strength does directly cause a lower quality of life. Capability and independence are vital to happiness. And yes, strength can be improved and maintained right up until death if you work at it.
Lowering your quality of life is far different from being the most important thing. A lot of things change when you age that work to lower the quality of your life; your happiness depends greatly on how you handle it.
Correlation is not causation (you might as well flip it around). And I'm afraid that even for the strongest person, they're will come a day where they won't be able to get out of bed.