Idaho is going the other way, the Senate just passed a bill that will constitutionally prohibit cannabis and any other illicit psychoactive drugs if voters pass it with simple majority in 2022.
We're talking about a state without big cities, a comparatively tiny population, generally not a lot of crime, and a mostly white population. The only thing we'd be able to learn is whether drug usage in that state is affected at all by not changing a thing (it's illegal now). This would indeed be surprising as doing the same thing over and over again and expecting different results is a sign of insanity (to paraphrase Einstein). Also, it would be a good way to characterize decades of drug policy making in the US.
It won't. I'm a legalization supporter, but you can't pretend that being surrounded by states with legal cannabis won't cause the availability of cannabis to be huge. If cannabis causes negative effects, they're going to get hit with a double effect of readily available cannabis and no tax revenue to offset it.
Then again, I don't get the impression that it's ever been difficult to find cannabis, even when it was illegal all over the US.
Which is great, for the following reason: a really good way to actually know the value of something is conducting A/B experiments.
If Idaho keeps cannabis illegal, we'll be able to actually measure what effects good or bad that has on society, including tax opportunity costs.