I guess it depends on where your from as to what people take and what they move on to.
ATM in the UK Crack's the fashion drug, and before that it was coke, and before that speed & Es.
No one specifically move onto another drug because they're taking a 'weaker' one - it's normally peer pressure or curiosity that does that. People can go looking for "the next high" (cliché) and find a harder substance.
Getting back to Marijuana a bigger problem with it's the amount of hybrid gear there is these days - all specifically designed to be stronger but also increasing the risk of personality damage. There's also the problem of resin getting produced cheaply with things like rubber mixed in.
I'm sure you'll all agree that the fumes from rubber aren't something that anyone should be inhaling.