The substance abusers don't want less of it, thats why they do it to begin with.
Address the causes that lead people into homelessness and addiction and you end up with less of it. That isn't conjecture, its fact, proven by dozens of disparate cultures and countries adopting or rescinding preventative measures with statistically traceable results proving positive correlation.
Where did I say anything about what substance abusers want? For what it's worth many of them want to get clean but can't pull it off because that's how addiction works.
My point was that substance abuse is (relatively) evenly distributed geographically and pretty much every city and state has more of it than they'd like. Homelessness to the point where it is a significant impact on the day to day lives of the non-homeless is a west coast problem (I've never had to step over human feces in DC, NY or Boston). I'm not saying anything about causes, effects or correlations.
You said "what we want" and one of the important things to remember when having a discussion about macrosocial policy like reducing drug abuse or homelessness is that it is exceedingly rare for everyone to be in agreement.
When confronting homelessness, remember that landlords, aristocrats, and even upper middle income working class all don't want to actually improve the homeless situation. To them its a cost center, and they save more money exporting their homeless by force than trying to actually get them housed and healthy. The policies currently in place out west and across the country, in relation to not just homelessness but all "problems" in society are all intentional. Its key to remember that, because recognizing the hostile actors in resistance to societal improvement is often half the battle in knowing how to make meaningful change.
Homelessness and drug abuse are both symptomatic of livelihood instability, as can be proven by how European nations demonstrated marked reductions in both as more policies are enacted to keep people from falling into extreme poverty. But neither is simple enough to distill to being the consequence of one decision or policy - the frequency of homelessness is influenced by many factors, including... 1) The livability of the area exposed to the elements (homeless in Seattle and Boston are often lower not because of good social policy but because its life threatening in the winter). 2) Local policy relating to the homeless. Mostly due to the age of many of the cities there are more well established non-profits, charities, and churches operating to assist the homeless in the East than the West. 3) Housing prices, obviously, are a huge factor. Its much easier to be livelihood insecure when your rent is a larger and larger chunk of your income. This is why, despite often rampant poverty in the South, homelessness isn't nearly as endemic because property is much cheaper in Nowhere Kentucky than it is in the Bay Area.
Address the causes that lead people into homelessness and addiction and you end up with less of it. That isn't conjecture, its fact, proven by dozens of disparate cultures and countries adopting or rescinding preventative measures with statistically traceable results proving positive correlation.