There are two factors at work here -- price and legalization, both inter-related.
Cutting the price of marijuana by 90% would, by itself, increase use. Probably double, or close to it.
Legalizing marijuana would, by itself, increase use, especially among teens. Probably double, or close to it.
IMO, we're looking at a quadrupling of use, maybe more.
In 1979 marijuana was neither legal nor cheap. Yet marijuana use was 3X today's rate. So, just a change in attitude could triple the use.
The second thing to consider is teen use. Marijuana is easier for teens to obtain than alcohol, yet teens use alcohol 2:1 over pot. Why? IMO, because alcohol is legal -- society has given it's collective OK for adult use. How bad can it be?
I don't agree with your predictions. It was my experience in school that essentially every person who wanted to try it, did so. So expecting a fourfold increase is not reasonable.
Second - suppose you are right and use did go up fourfold. What would be consequence of that? Teens get killed in drunk driving accidents a lot more frequently than in pot-related incidents (that is, where pot is a primary or contributing factor, as opposed to merely being present). So I don't accept the end-of-the-world-as-we-know-it scenario that you are presenting either. For all you know, some teens might stay away from alcohol, and not get killed in crashes.