It's no different than "banning abortions to reduce the amount of abortions". It doesn't work that way in reality, and rather than face reality, they dig in deeper, stick fingers in their ears and go LALALALA CAN'T HEAR YOU. Except that NiMBYism goes strongly across both major political parties.
Though there's plenty to say about "affordable housing", many have argued we just need drastically more housing rather than a lot of government regulated housing prices. We're just not building enough in places people want to live, specifically because zoning (looping back to NiMBYism). [Andrew Price](https://andrewalexanderprice.com/blog20210125.php) and Strong Towns have plenty to say about this.
The reason cities are struggling with homeless populations isn't because "treat them like people with dignity" doesn't work, but rather that there are MOUNTAINS more homeless people than most cities fund helping. This is exacerbated by housing being stupidly more expensive than it has been in the past in cities. Homeless people don't have a home. Plenty of them have jobs they are desperately trying to keep. But when the average income in your city is less than the average housing cost, you inevitably get way more homeless people.
Also, the point of decriminalization of drugs is just to reduce unnecessary harm, since our justice system sees anyone in it as expendable and unwanted. If you want to actually help homeless people get off the street and back into society proper, you need to fund enough shelter beds for them, enough social workers such that they know these people by name and have the time to actually work for them, and some sort of jobs or education or enrichment programs.
I’m all for affordable housing and work programs and other things that allow those who have fallen on hard times ways to help themselves out. I’m also for psychiatric care for those who have mental health issues that won’t be able to help themselves. None of that involves safe spaces for recreational drug use. That only keeps people on their state or drags them down and provides incentive for others to follow (as evidenced by Seattle, Portland, SF, LA, etc.).
There are lots or organizations that will give people a place to stay, especially woman and children. The good ones will also have educational help and employment services as well. None of that needs to include trip sitters, needle exchanges, narcan, or paraphernalia vending machines.
Last time I checked San Francisco spends something like $50,000 per year per homeless person. It's not a lack of funding in some places.
Legalization (or decriminalization) is a possible pathway to supporting people, but the current method of helping people with myriad issues stay in their current state in life (or worse, encourage more to enter it) is one of the biggest public health disasters facing urban areas. Major cities are being gutted by this failed line of thinking.