"Could be that instead of 10% of 5 million illegal marijuana users going on to hard drugs, we'd have only 5% of 10 million legal marijuana users going on to hard drugs."
Who knows? Maybe less, maybe more. This is all supposition and speculation with little if anything to back it up.
"Would you support the legalization of all soft drugs?"
Possibly. Define "soft drugs." MJ, and what else? LSD? Ecstasy? Mushrooms? What's the dividing line between "hard" drugs and "soft" drugs? How addictive they are? None of the latter 3 are known to be addictive in the classic sense of the word. I would actually argue that MJ is mildly addictive, in as much as there are mild withdrawl symptoms when chronic users stop. Mostly the "addiction" is psychological, to the high. Actually I believe that is true of most drugs, with the exception of alcohol, where for alcoholics sudden withdrawl can have very nasty effects.
So, to answer your question, yes, I probably would support the legalization of all "soft" drugs. In theory I support legalization of all drugs, but I'm not so sure about how it would work out, especially with our modern culture of drug-taking. I mean, up until the later 1800s or so, most all drugs were freely available in multiple forms to anyone who wanted them and could pay. But then there were also many people who were addicted to laudunum, cocaine, and so on that was contained in many patent remedies.
Marijuana, club drugs like Ecstasy, GHB, Rohypnol, and Ketamine, LSD, peyote, shrooms, nitrous and other inhalents, and amphetamines.
"What's the dividing line between "hard" drugs and "soft" drugs?"
Probably the degreee to which you can become addicted. But usually you have to ask, as you did, what people mean when they say "soft" drugs.
Quite frankly, I don't see how anyone can make a legalization case for marijuana and exclude the other drugs listed above. Marijuana, in my opinion, would be nothing more than a first step to the legalization of the others -- the same arguments could be (and probably would be) used.