The situation is weird. No one thinks like this, or it isn't a problem, in cities that actually have good public transport. A drug addict has never entered my mind when riding public transport in London, Paris, Sydney, Tokyo, etc.

IMO the root cause of problems you guys are having is internalized classism and strong affections with the idea of natural selection. You can have a homogeneous society with good welfare and not worry too much about spoiling ourselves, a near or even actual communism with extremely subsidized educations can still have bloodbath levels of internal competitions. You can do that and then you can make it weird for tons of over/under class people with absolutely no way out to be just there in plain sight.

I live in the heart of the Mission and I endorse that quote from the parent comment 100%.

Once every few days I even share public transit with a homeless person! Oh, the horror!!

The people I see out on the street are suffering terribly, sick, unable to escape the gravity of their situations, and the vast majority are not malicious. Anyone who does not see this has chipped away key bits of their own soul.