There are two things here to challenge.
1/ is it absolutely maybe has. Just because something was out of sight and out of mind before doesn't mean the problem was less prevalent. It all depends on the definition of the problem.
Is the problem: "There are people in poverty, in drug addiction, and they're dying?" Or is the problem "Sensible moderate tax payers have to interact with icky drug addicts in their day to day life"
The truth is because of Fentanyl both problems actually got worse. But in theory treating addiction as not a crime probably DOES save lives relative to the alternative.
But make no mistake, we're not now challenging the status quo because of problem 1 and the fentanyl epidemic. There's a backlash becaue of problem 2.
Truth is, people don't want to see visible poverty or addiction. They associate with crime and feel unsafe (even if it's illogical).
People agree that you shouldn't have to go to jail for doing heroin, but when faced with the choice of passed out heroin users in their alley regularly, or have them in jail, most are happy to find the excuse to get them out of their sight and into jail.
So 2/, the real reason decriminalization didn't work is because decriminalization isn't enough. Decriminalization is "cheap" to implement. But it's nothing without more social programs to help people get out of poverty, subsidized housing to get them off the streets, and mental health support for those that need it (a huge % of homeless addicts). But nobody - even in progressive places like Vancouver - wants to fund those enough. So, we try decriminalization, watch it fail, and then roll things back. And so the pendulum will go back and forth.