As a 24 year old software engineer who has smoked weed daily for the past 5 years and, as an individual, earns more than the average household income, I disagree with your statement. I know many other people who smoke as often as I do and are also arguably successful people for their age.
Yes, most stoners I know work near-minimum-wage jobs, but from what I've seen, this is because underachievers and unmotivated people are drawn to weed, not because weed necessarily makes you an underachiever or unmotivated. It does make you relax and more at peace with the world, which can lead to lower motivation (why exert yourself when you're already content with things?) and just wanting to chill, but it doesn't take much effort to keep yourself motivated and on track while maintaining a weed habit. In fact, smoking some weed after work is a great way to unwind, release stress, and prevent burnout.
Smoking weed doesn't make you stupid any more than drinking alcohol will make you abusive or drinking caffeine will make you a tweaker. Which is to say, it can if you let it, but it's pretty easy to avoid it with a little effort.
That said, I can certainly believe that smoking weed isn't a good thing for kids since they (in general) don't have the maturity to use responsibly nor the established lifestyle habits to prevent complacency in the face of weed's tranquility.