As a scientist I dislike the trend of observational studies getting massive press (see associations between Wine, Coffee, Chocolate etc and health/lifespan) because, as you say, we are doing (and should be funding more) studies on some of the underlying mechanisms but it is difficult to summarize them in a punchy headline.
There are have also been some really elegantly designes observational studies that use comparisons between family members, twins or features of the population to reduce bias in an ethical way.
Arguing that something is true because it is too difficult to research the subject properly seems just wrong on so many levels.
I find research ideologies like this at its very core highly dishonest. Sure, people who are really into the subject and methodology understand a study's limitations. But all it does is leading on the media to hype one week the story "A makes you live longer" and another month later "A makes you die faster".
You need to understand that a clinical trial on this matter is nearly impossible.
1) Imagine the cost of holding a clinical trial over 20 years.
2) Imagine the difficulty of trying to get people to adhere to a workout routine for 20 years!
3) Given what we already know, it's probably unethical to tell a control group not to exercise or build significant muscle strength.
4) Finally, even if you work out all the issues above, imagine trying to get funding for a hugely expensive study trying to answer the question of whether strength training lowers these risks when it's already widely regarded as healthy.
The researchers are very aware of the limitations, so they went to great lengths to exclude the most likely confounding variables.
> there are also confounding factors like genetics and selection bias
To be more accurate, these are potential confounding variables. It's very possible that the association would hold up if we could adjust for these variables, too.
Causation here is not proven, that's true, but taken together with everything else we know, it does strengthen the case. E.g. we can independently explain the mechanism whereby the heart is strengthened by resistance training. I'm not an expert in either cancer or heart disease, but I would love to hear whether we can explain the mechanism of action already.