This was true thirty years ago, but is much less true today. The fraction of people that have serious mental illness is relatively constant, but the number of visible homeless people on the street has risen dramatically. What you're seeing today is a fraction of mentally ill people and then a larger fraction of drug addicted people stemming from the opioid crisis.
It's not the 80s anymore and that person screaming to no one on the street corner is more likely to be in meth psychosis than schizophrenic.
The reason I point these out is because I think the different groups need different solutions. Affordable housing isn't going to do anything good for something in the middle of meth psychosis. They'll just trash the place which is a complete waste of funding.
Don't confuse the two. Breaking down a working system that was evil without creating a replacement, without gradual transition, can (and did) result in a more evil system.
They were motivated by compassion but not right minded.
The reason people keep looking to law enforcement to solve these problems is because they can. Take away that and institutions (government or otherwise) will eventually spring up to solve the problem. Visible homelessness running rampant in the time it takes for that to happen is simply the penalty communities will have to suffer for being so economically dysfunctional as to have the problem in the first place. This is the phase some west coast cities are in now and of course there's some backlash and people who want to go back to strict law enforcement.
As long as someone acting on behalf of the government can tell a homeless person to move along from a public space the problem will not go away. It will just move to the next street over for a few hours then come back when it gets kicked out of there too. These people need either better options that are so much better they take advantage of them (i.e.) better shelters or to be institutionalized against their will and I don't think the latter is compatible with our values.