The evidence shows that housing-first policies applied to the worst cases (chronically homeless single people with addiction) is effective and lowers costs
I would like to dispute that. This WaPo [1] article paints a far less rosy picture.
Addiction is a symptom of mental illness (and in all likelihood may be genetic). Look at the rates of smoking among people with schizophrenia [2], they're much higher than the general population, and the usual purported causes fall flat.
[1] https://www.washingtonpost.com/local/dc-politics/dc-housed-t...
[2] https://slatestarcodex.com/2016/01/11/schizophrenia-no-smoki...