Decriminalizing drug use is intended to solve a different problem - that being throwing people in jail for being addicts is bad in a number of ways and consequences.

Making all drugs legal and available (with quality assurance) will (at least help) solve the problem you are talking about, but will at the same time create whole new problems that more and easier access to drugs will mean more people try and use them and then an increase in lives destroyed through addiction (and related consequences). My opinion is that would be worse.

Decriminalizing use basically decriminalizes selling as far as I can tell here in the northwest. Police and prosecutors can no longer threaten users with legal action to give up their dealers, and since buying is now low risk, dealing also becomes lower risk. I’ve seen people on the street looking for drugs and find them on my morning walks (they are pretty vocal when asking, and dealers aren’t shy about replying).

It would be easy to put out fake buyers to find the sellers. Sounds like lazy policing.

It is hard to find police that can convincingly do unhoused drug addict well enough.

There are test kits available that can show you what you have, including tests for fentanyl. It requires a bunch of different reagent tests if you bought a Mystery Powder that you want to ID. But if you were after MDMA, for example, and bought what’s claimed to be MDMA, you can test it with a specific reagent to confirm whether it is that or not, then test it with a fentanyl test strip to show if it’s free of that contaminant.

Dance Safe sets up tents at big festivals to do this kind of testing as a harm reduction service. They (and others) are trying to normalize the step of testing what you have before using it, and I think that’s a wise choice especially with fentanyl in the mix. I’d expect that with decriminalized use of “white powder” drugs, you’d see these kits become more readily available from smoke shops, maybe even pharmacies, instead of being something you have to search for online.