>>>Losing a lot of lean mass is incredibly detrimental to your longevity and quality of life.

While true, its also true that if you manage to lose substantial fat in the process, it leads to longer and better quality life

> it leads to longer and better quality life

This needs a slight change in wording or clarification, depending on what you meant.

Losing substantial fat when overweight increases your chance of a longer and better quality life than if you had maintained high levels of fat. Losing substantial muscle in that process reduces your chances of the same. It's statistics and never guaranteed.