People high on pot never noticed that the Grateful Dead often played out of tune.
In my career I've been involved in the development of hundreds of drugs. When Magic Johnson announced that AIDS wasn't going to kill him in the near future, felt a special joy, because I was involved.
Why am I constantly badgered here about finding something positive to say about a recreational drug?
No one expresses so much passion for sildenafil or prospecia?
Do I have to spend oodles and oodles of the last years of my life looking for something that I feel has already wasted too much attention?
If your old man was actually right about a topic, and I repeat his correct statement 40 years later, even if he stumbled into being right with no real information, whether or not this annoys you had no bearing on whether his statement is or was true either then or now.
If one searches on Google Scholar using the words Cannabis adolescent and brain, one can get 47,900 hits in 0.11 seconds. I'll bet that many of the authors of these 47,900 hits agree with your late father, as do I. Some of them certainly had spectacular educations, other less so.
I'm not sure if they'd annoy you, but anyone who really wants to spend their time and lives thinking all about pot, can certainly do it.
I have other things to do, some of which are involved with saving lives.