And from my experience your observations are not the same as my observations.
I’ve seen a few turn into parasites, but most heavy long term users who began in their teens actually turned out to be “normal” in the end.
I suppose it really comes down to the individual and their personalities. I don’t feel that those who end up on the low end of the ladder of success do so strictly because of heavy pot smoking.
Think of the bus driver on the Simpsons. They were into their music, being high most of the day and avoiding anything that interfered with hanging out and being mellow.
After a few years of living like that it was all they wanted to do.