If the only thing the WTS did was have a few beneficial rules regarding health and good living, that would be great. But they also prohibit blood transfusions. They encourage shunning of those who are disfellowshipped. They demand --under the threat of being disfellowshipped-- that the leadership be obeyed unconditionally, even when they are wrong.
If every JW followed this advice and was a paragon of clean living and good health, those latter rules would still be a reason to leave the organization. No one gets disfellowshipped for being obese. They probably don't even get any friendly counsel. The rules they enforce the most are the ones that make the organization such a bad place to be in.
Is the occasional success story worth the suffering that their policies inflict?