While that's true it's also a big reason to promote fitness. My obese sister has the same health issues as our mother who is twentyfive years her senior. 

Being healthy and in good shape for your age is it's own aspect of beauty. The whole subject is quite multidimensional; a lot of people don't feel sexually attracted to very beautiful people because of behaviour, the persons age or a type preference. 

That someone cares a lot about having a relatively lean partner doesn't mean becoming wrinkly will make them fall out of love.

When a partner loses his looks and health because he's neglecting himself it's not just aesthetic decline that can put off a partner but the entire dynamic around it. There is  being physically disgusted by something (like a bad smell) and emotional disgust (like when you know someone refuses to shower).