This is an interesting and obviously scary topic.
For starters, none of this holds water statistically. Performing a brain autopsy on dead, former players tells us very little. Where is the control group? What other factors were there (other sports, car accidents, abusive parents, drug abuse, genetics ect ect ect).
Obviously, Alzheimer (et al,) isn't reserved for former players. What's the baseline. Unfortunately, most people do not have autopsy's performed on their brains, so we can't measure the effects.
A lot of this issue is fear-mongering. Some of it is backed up by real (scary) evidence.
Interestingly, HS basketball has a higher rate of concussions than football. The NFL rate is something around 10% of players (per season). The high school rate is about 5% (no data for college). High School Basketball is right around 7-8%. Naturally, a football team has a lot more players, so there are going to be more total concussions, but as a rate... basketball is touch higher.