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.
Those you describe would probably have been the same type of people regardless of their use of pot in my experience.
You may know losers like the ones you describe, but I know users who would burn through an unbelievable amount of pot on a daily basis and still were motivated, intelligent, and productive people.
Your stereotype simply fails to accurately depict the majority of heavy pot smokers I've personally known over the last 30+ years. It may accurately describe your observations, but I was very close to those I am describing and understand them very well along with what other recreational drugs they imbibed in their adulthood, and I don't know if you're as intimately familiar with the people you describe, so I can't say that our observations are comparable.