Sonia
You raise interesting points here, and the key challenge is, as you note how can we evaluate these interventions within a reasonable time frame and a feasible amount of money? The most convincing opportunities are probably natural experiments: when someone implemented a policy change, it provides an opportunity to assess whether outcomes improved after the implementation. You might imagine a country in which such improvements were implemented in a staggered or randomized fashion so we could really understand the health consequences, but that is not typical in the US, even when there is not enough funding to implement a policy universally. As a result, people look for natural experiments to provide evidence.
Maria