I've seen evidence of both. I've known some big-time potheads who ended up (1) owning their own company after being president of another or just plain out (2) succeeded in a decent career, grew up and matured and also (3) ended up being a bicycle messenger and at age 51 that is still what he is doing. He still looks and sounds like a total burn-out too.
I think it depends on a lot of factors but I won't dispute that the likelihood to be #3 in the above scenario is greater with abuse of pot.
1 - Cannabis is not strongly addictive and if a person chooses to lay it down - it's not that hard to do.
2 - If someone gets snagged up in the judicial system in regard to cannabis use and they are prone to doing mind altering substances - they may move on to less noticeable substances. (That - to me is a real bad thing about the prohibition of cannabis.)
I've been wanting to use this for a while -
Keep your laws off my bong!