Well there are a whole lot of problems with your assumptions. You implicitly assume weed is more dangerous than alcohol or tobacco. This is medically proven to be false. You also implicitly assume that legalization has a huge effect on supply and demand. Legalization affects criminal enterprises and price. You legalize, criminals stop dealing because it is no longer profitable. Does making meth illegal stop people from trying it? No. Does making it legal cause more people to try it? No. Cigarettes were illegal to me until 19 but I didn't become a smoker the day I turned 19. Same for alcohol.
You really think that if drugs were legal, people wouldn't try them more? Why do you think almost everyone's had beer, but not as many harder drugs? Eh?
It's cause they aren't available to pick up and give it a shot at the 7/11.