Let's put causal relationship aside for a while. I agree that a causal relationship is hard to prove. Even now, you still can't say smoking is the cause of lung cancer. It is a risk factor for lung cancer. But there is no doubt that it will reduce lung cancer by quitting smoking.

When you find a significant association, you conclude that there is an association by admitting that you could make type I error and your conclusion needs to be confirmed by other studies. If enough evidence is shown that near work is an independent risk factor for myopia, then it is worth to do further research and possibly a RCT. This is a common practice.

The controversy is "Is there enough evidence to recommend some preventive actions in order to avoid the development and progression of myopia?" I would like to learn more about that. As someone who has high degree of myopia and don't want it to get worse, I choose to reduce as much near work as possible unless there is compelling evidence that it is useless. I would give my kids the same advice instead of telling them near work is not causal even if there is an association.