If by decriminalization you mean replacing prison time with mandated treatment, I can support that for non-violent offenders as long as there is a clear 'red line' after which (part of) the present prison sentence is applied.
No, I don't think people that can't stop doing heroin should be put in prison, unless they're doing violent sh**t or robbing people, but then only for those things, their addictions shouldn't play a role in the length of their stay.I don't know about massive increase, maybe for the "lighter" drugs like mdma and weed (and even then, not massive increases...if you want to get high now, it's not hard at all to find it....despite putting millions in prison, the war on drugs has done NOTHING to stop the accessibility of drugs), but you're not going to have regular folk going out and becoming coke heads because they can suddenly go downtown and buy some at the store.
There's some people though who want to get high but don't because:
A) fear of drug dealers
B) fear of prison and/or
C) lack of information about accessing the black market
We don't know how big of a group that is, and 100% of them would start getting high the moment they can just walk down to their local pharmacy and grab some. Also, existing users might start using more than usual due to easier accessibility.
These are legitimately dangerous drugs, and to legalize them is only to encourage their use. And if that means throwing those who don't respond to treatment in a reasonable amount of time into prison, the government must continue to do that.