It is not speculation, it is observation and experience.
There are people who smoke who have successful lives if they have the self-discipline to not let it interfere with their work and their family. I said that.
Most people who start using regularly in their teens and are heavy users for years have low level jobs and do little but hang out, get high, go to the bar and things like that.
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.