The only way to diagnose CTE is to use an autopsy. It is obvious that there are many, many young men being damaged by brain micro trauma. 
Your letter is kind of silly. It is saying there is no danger from lung cancer from smoking because kids rarely get lung catastrophic injury. The second is true, but it is not in any way related to the first. 
Any parent who cheerfully has his or her kid play gladiator sports like football is either ignorant of the latest studies, or a really bad parent. And don't tell me small sample extrapolation is bad science. It is, in fact great science if it is the only way to get the evidence needed. 
Hugh