Your mistake is thinking it is an either/or thing. He fried his noodle 6 ways to Sunday on Acid. Among other recreational pharmaceuticals. Today, 50 years later he has Dementia on top of it.
Is it your contention that LSD would be good for Brian Wilson right now? Good luck with that, Doc.
I don’t believe LSD fits into the known causative pathways to dementia. It can, however, be connected to some of the schizoaffective symptoms: that is common.