Most people can't be bothered to exercise regularly even though it generally means a significantly longer healthy lifespan and higher quality of life. I'm pretty skeptical that saving 20-40% on a gym membership is going to create any real marginal benefit in terms of population-level fitness.
If you want to improve peoples' health, you need to move away from what has caused the problems in the first place; average North American food is generally an abomination and the SFH/suburban environment most of us live in isn't conducive to a naturally active lifestyle. But changing either of those things drastically at this point is a massive undertaking, and even just advocating for things like walkable, compact cities/neighborhoods riles up conspiracy theorists enough to lead them to threaten the lives of the people who push for them because they think the WEF is going to put them in a concentration camp.
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