> good health is the best prescription for preventing chronic diseases
That's bad enough to approach self-parody, but in a way it's very illustrative of the fact that apart from smoking cessation and arguably some other forms of drug addiction treatment, we have very little in the way of demonstrably effective lifestyle interventions to prevent chronic illness. There are mountains of epidemiological associations and sometimes even plausible physiological mechanisms for many of these things, but it often turns out that the obvious intervention just defies the correlation instead of changing the outcome (cf. Goodhart's law), or changes things in a statistically significant but practically underwhelming way.
As far as I know, obesity is mostly significant as a public health issue because it makes many conditions modestly worse, not because it's the primary cause of any given problem. Losing weight might mitigate an illness such that you cross some diagnostic threshold and are thus "cured", but you're still in a much different state than someone who wasn't at risk of that illness at any weight. That doesn't mean you shouldn't try to maintain a healthy weight, just that it's wishful thinking to expect that doing so is going to solve all or even most of your problems. In general, it's important to recognize that health is a complex topic and not fool ourselves into thinking we've found the key to good health.