I think a lot gets missed in these types of comparisons with the US because the US is both a much less urban country, and also a federal system where a lot of problems get pushed down to local governments.

These two forces work in opposite directions of attraction. Because the country is less urban than Western Europe, the most attractive places to live are not exclusively the big 3 cities. Our wealthy people are pulled away from the cities as much as they are attracted to them.

Meanwhile, a lot of national problems - drug addiction, homelessness, mental illness.. are essentially handed down to cities to deal with. Blue cities by and large have more social services, shelters, lax drug laws, needle programs, etc.. which attracts people who have these problems. Further, some cities like NYC have laws on the books that essentially force the city to provide a shelter bed for anyone who wants one.

Right now we are seeing this made worse by NYC absorbing a disproportionate number of migrants which are then the cities responsibility to house/feed. Wherever you stand on migrant policy, NYC has received a number of migrants equal to the pre-existing number of homeless shelter beds, at a time when shelters were at a an all time high utilization. So our homeless/shelter budget which is already higher per capita than most of the rest of the country just needed to double.

So blue cities end up with disproportionate number of problems they need to solve.

Portugal introduced mandatory treatment along with decriminalization; that's the component that diverges quite substantially from North American policy.

I think the biggest difference here is that Portugal abolished prohibition, acting with unity as an entire country. That's very different than when cities or states do away with it, but there's unrestricted travel between there and thousands of counties next door or a few states away, where prohibition is in full force. Those counties get hit with not only an "on-steriods" version of the symptom in question, but also any other demographic patterns that are corrolated with it for any reason, such as (in the case of IV drug users) extreme poverty.

One frequent response to this point, is for someone to say "I'm open to decriminalization arguments, but I'm not supporting anyone who says 'The entire country has to adopt my policy in order for it to work!'" Which sounds reasonable, on its face, but in this case it seems like it really is the best option.

I'd like to know some timelines on that, cursory googling get's me Portugal decriminalized in 2001, Oregon only did that in 2020. Systems don't change on a dime, how long was the transition period for Portugal?

Not necessarily... "niceness" has a lot of contributing factors. If we're cherry-picking, Singapore is pretty "nice" at street level.