Most people are near-totally uninterested in their actuarial life expectancy. As a result, close to 100% of people on a diet are looking for weight loss.
Whether you should consider it a "health" objective is a more interesting question. From a Darwinian perspective, attracting a mate is much more important than whether you live to be 67 or 72.
And yeah muscle tissue is the important one. Any diet is sustainable until you burn through all your secondary reserves. How many buff vegetarians do you see? In my experience, you need to be a bona fide nutritional expert to maintain muscle mass on an exclusively plant-based diet.
As buff vegan the proportion of buff vegans among vegans is probably the same as proportion of buff non-vegans among non-vegans. You have to be braindead not to be able to figure out what to eat as vegan to grow muscle, in the worst case I guess you can go to factory farmers who have mastered the art of growing muscle from plants..