I don't pretend to know your station in life, or your experiences. I can only talk to mine. Having seen my grandfather gradually succumb to dementia is something that will stay with me forever. This man built houses for forty years. At times, quite literally by himself. Farm stock from Ohio, never depended on anyone for anything a day in his life, until suddenly he couldn't feed himself anymore. Fell in the bathroom. Stopped being able to drive. Couldn't sign his name to a check, lost control of his finances, accused his oldest son of stealing money (when in truth, my dad was paying for his care from the bank account that he'd been given stewardship of). HIS BRAIN CHANGED. My grandfather wasn't an asshole. He saw both of my kids born, he was Santa Claus at Christmas, he traveled thousands of miles a year to see our family in Ohio, North Carolina, Tennessee, Florida and Texas. And at the end, he wasn't that guy any more. HIS BRAIN CHANGED.
Another family friend was textbook alzheimers. Assistant chief of police at the University of Texas for decades. He probably arrested and/or lectured SEVERAL people on this website, in fact :). This man couldn't remember the names and faces of his own children and grandchildren. HIS BRAIN CHANGED.
CTE changes the brain. It's selfish, short-sighted, and irresponsible for you to equate CTE with "oh, that guy was just always an asshole" because you like gladiator circuses every Sunday.