Strawman fallacy on your part, but that's irrelevant. It wouldn't require opening the brain of every member of a study to gather information. CTE sufferers exhibit certain behavioral changes. Study the behavior and see if naturally aggressive males, the ones who gravitate to football, have higher rates of depression, anti-social behavior and suicide. There are many ways to get valid information instead of running studies that confirm what you already thought and wanted to find.
The CTE studies are guilty of post hoc assumptions--after this, therefore because of this. Just because some reputable group runs a poorly designed study and trumpets their findings doesn't make them right. A large segment of society would love to see the downfall of traditional male activities and will not dig too deeply into the study design if it confirms what they just had to know was right.
A study doesn't have to be an experimental design in order to be "good science" even if that's the gold standard. Most studies do not use experimental designs, and even the ones that do take liberties with their assumptions (for example, much of biomedical research assumes that mice are physiologically equivalent to humans). There are methodologies to account for factors beyond the scientist's control. It's literally why statistics exists as a field.
The funny thing in your post is that what you propose studying with the cohort of children would be bad science. A study that assumed anyone who exhibits aggression is attributable to an underlying condition of CTE versus any one of a hundred other explanations would get thrown out by any publication immediately. It also would never get IRB approval. If you wanted to measure if kids who play football are more aggressive, with some operationalized measure of aggression as the dependent variable, then that could be done. But you also couldn't then say it was due to CTE except as one of many possible causes. Then people would take issue with how you measure aggression, etc.
Currently, the only way to diagnose CTE is posthumously. You pry open someone's skull and look at the tau deposits and it's pretty obvious. 27 year olds don't have severe tau deposits from Alzheimer's or normal aging processes. There's a pretty clear correlation between suicide and CTE and you can study that statistically. You can't prove any counterfactual of course (what happens if Aaron Hernandez never plays football), and you can't say for certain whether CTE is the proximate cause (people, after all, kill themselves for any number of reasons). So you have a correlation, and a strong correlation indicates a risk factor or symptom. Not all colds cause sneezing, and not all sneezing is caused by colds, but sneezing is one possible indicator of a cold.
In any case, we have a pretty good idea of what CTE does without needing to track people with CTE, since at high ends it's very similar to Alzheimer's or senile dementia. Both of these are known to cause seemingly random bursts of aggression or anger, as anyone with a family member suffering one or both will tell you.
We have a fairly sizable proportion of the brains of all NFL players who died in the last 8 years or so. Believe me, I'm skeptical of a lot of very crappy science, and a lot of studies that are trumpeted in the papers and on TV are awful studies by charlatans (don't believe anything anybody says about diet and nutrition, for example). But this isn't crappy science. The studies are literally just chopping up dead brains and looking at the tau accumulation. It's the tau accumulation that causes the symptoms; and, as noted above, we have years and years of Alzheimer's and dementia research for that.
