That's a good point, but if you're talking about addiction, mental illness is a precursor to addictive behavior. It is one of the major reasons why people self-medicate. Certainly is not the only reason.

My philosophy is: certain things are good for everyone if everyone has them. Some of these things: food, education, healthcare, place to sleep (securely), modest personal possessions, and so forth.

Reasons are mostly because of what you said. Homelessness leads to a general decline in society. If more people have healthcare, the general health of everyone goes up. If more people are fed, more people are productive, and production is good for everyone (to draw an incredibly broad generality). And so on.

It's unrealistic to say we'll do all those things 100%, but I do not think they are unrealistic goals for a modern and technologically advanced society.

Price breakdown:

30bil/year to end world hunger. 175bil/year to end world poverty.

20bil/year to end homelessness in the US.

I can't find numbers for everything, but those are some good ones to think about.