This has been an increasingly popular argument here in Portland since decriminalization. It's deployed, generally, in terms of re-criminalizing hard drugs until there's a well thought out framework for safe, regulated legalization.
While I agree in principle [edit: let's say I did agree, but my views on this subject have shifted radically since I voted in favor of decriminalization - and I'll admit I was naive and wrong], I think that while decriminalization without regulation is clearly catastrophic, legalization with regulation would also not be desirable so long as it's confined to one local city, county or state, in the midst of a nationwide fentanyl crisis. Portland simply does not have the capacity or infrastructure to accept further waves of addicts from all over the country who come here to live on the street. Legalization means more regulatory burden, more services for out of town addicts paid by a dwindling local tax base that's quickly being displaced and/or opting to leave.
To do that experiment and do it right, it needs to be nationwide. In any case, Portland can't go it alone anymore.
And remember - we did have a long experiment with legally prescribed opioids, and their widespread availability contributed to the current addiction crisis.