we should be genuinely doubtful about how much exercise will help with non-athletic or cosmetic things.

First, we probably should be interested in the amount of total physical activity -- "exercise" implies additional activity besides the baseline and the baseline varies a LOT. Some people work as lumberjacks and some people only move between the couch and the fridge.

Second, as long, as we are expressing wishes about studies, I'd like those studies to focus on differences between groups of people (e.g. run some clustering) and not just smush everything together into overall averages.

Third, there is one more category besides longevity and (athletic and/or cosmetic) -- quality of life. Being fit noticeably improves it and being out of shape makes it worse.