According to this site[1], felons incarcerated for drugs comprise 16% of the total prison population. 64.8% are incarcerated for violent offenses and property crimes, both of which have victims.
The most interesting part, to me, is the "public-order" criminals. When my girlfriend worked corrections, she noted that at the state level, the vast majority of prisoners were in for violent crimes - rape, robbery, murder, arson, etc. At the county level, however, a large number of prisoners were in for what she termed as "criminalized insanity." They're too mentally ill to function, so they live on the streets until they get arrested for doing some small crime - loitering, public urination, trespassing, etc. They get locked up for a few days, get a court date, and get released. Then they go right back to the streets, miss their court date, get arrested again for a bench warrant next time they have a run-in with the law, get sentenced to a slightly longer stay, get released on probation, violate probation, get arrested again the next time they have a run-in with the law...
Wash, rinse, repeat. All of this is really short-term... and yet they're forming 13.8% of the incarcerated population. We're spending an enormous amount of resources on what amounts to impromptu, ad-hoc, half-assed mental healthcare, and none of it works. But eliminating it entirely and saying, "Go forth and sin no more" is going to do absolutely nothing unless there are corresponding programs that can actually help such people.
That kind of coordination - saying, "Okay, we're going to have to spend $x million extra dollars extra for a few years until the mental health benefits start to decrease the cost of jails and prisons" - is extremely difficult, if not impossible to accomplish.
Regarding drug pushers and prison - I constantly swing back and forth on this issue, mostly because they are morally degenerate for exploiting human weakness for profit. If you google Whiteclay, Nebraska and go to images[2], you'll find a whole bunch of pictures of wasted Indians lying in alleyways right outside the (white-owned) liquor stores. It's created an enormous amount of frustration in the community, and the response from the government is "Well, it's not illegal to sell alcohol to alcoholics."
I give pause to the folks who demand unrestricted drug use for everyone, (which is what decriminalization is - if something's restrictions have no teeth, namely incarceration, it's unrestricted. Same exact reason why financial laws are flouted so often - no teeth) because this is the natural result - businesses that exist solely to profit off of human pain and despair and create it on an industrial scale.
The real question, and one that I can't even begin to unravel because there's so much goddamn bullshit on the Internet from agenda-pushers on both sides, is "Is this state of affairs, where a large portion of the population is incarcerated for vice, preferable to a state of affairs where vice is openly permitted?"
Personally, as someone who lives in a relatively affluent suburb, I'd say that legalizing everything would be a massive boon to me. My taxes will go down, and more importantly I'm insulated from the consequences by a lack of public transit and vigilant cops who are, er, unfriendly to the lowlives who occasionally wander away from the city. The folks in the city and poorer towns, who actually get to deal with the massive consequences of widespread addiction, might beg to differ.
[1] http://felonvoting.procon.org/view.resource.php?resourceID=0...
[2] https://www.google.com/search?q=whiteclay+nebraska&client=ub...