I see what your saying that maybe another drug may take its place and be the most common drug (gateway drug maybe even) but i in my opinion don't feel it would increase sales of drugs that are more severe, i feel it would decrease people trying bigger more worse drugs, because marijuana is the main gateway drug.
Everyone i know that does severe drugs started with marijuana, and i feel if marijuana where to become legal it would become no longer a gateway drug (after 10-20years or so) because it wouldn't be looked at as a drug really anymore, so i don't feel kids would wish to start with a drug that would be looked at as severe, because at the moment, kids start out afraid to try marijuana, they drink than they try marijuana than since marijuana was ok they try something more severe and get addicted, but the kids i don't feel in my opinion would be willing to just start and try crack or heroin without experiencing marijuana first, they take 1 step than another, of course once marijuana would be legalized (if it where to) it at first marijuana would still be looked at as it is at the current moment, but after about 10-20 years it would be looked at as less of a drug, so kids woulnd't feel as safe about trying more dangerous drugs after experiencing marijuana.