I dont really believe in the premise that people smoke weed because its illegal.

The biggest group you will find doing things simply because they are told not to would be teens. Its a naturally rebellious age as well as an age of experimentation.

If weed were legalized it would get mostly the same restrictions as alcohol or cigarettes, so it would STILL be illegal for those who do so because its illegal. Therefore, most likely no shift to harder drugs would occur due to its legalization.

You might see some slight increase in usage if it were illegal because you also have a group (most that are beyond teen years) that will not do it because its illegal and will not risk consequences of illegal behavior (risk of job loss, drug testing at jobs, risk losing custody of children etc).

If we look to the one nation its been legal in for decades now we see that they did have a slight increase in usage after legalization but that was followed by a decrease. You also dont see a shift that means higher usage of other drugs either, that usage is also a bit decreased as well and still remains much lower than the US and lower than most of the rest of europe as well.

Possibly the reason is that their education and treatment of the issue is more honest and the funds they allocate for prevention and treatment is more effective in garnering less usage than criminalizing is. They dont have to spend quite as much on law enforcement, legal proceedings and incarceration...that gives them a distinct advantage.

Overall, I dont think most people continue drug use or drinking because they wish to rebel or even try it due to that reason...mostly the drive is to change our state of mind..humans like that and always have.