>The obvious answer is supervised injection sites where people can safely shoot up, which would then allow public restrooms to reopen and not be constantly occupied by people with needles.

Are you sure about that? Having traveled around Europe and North America, the only public bathrooms that weren't absolutely filthy were the ones that you had to pay for or were privately maintained (e.g. mall bathroom)

So I'm not sure that's the obvious answer.

Also, in your 3 steps you just assume that the level of homelessness in LA, San Fran, and Seattle is normal - and it isn't. Those municipal governments are terrified of crazy activists and have completely abrogated their responsibility to the rest of citizenry and to the homeless. The reality is that homelessness in those cities are wholly due to mental illness and/or drug abuse. The laws are so bad that even immediate family cannot forcefully institutionalize their family member and take them off the street so they can get the treatment they need. According to ACLU and activists compassion means you let those individuals (who cannot make decisions for themselves) languish in filth on the streets.