Drug use problems. Not drug gang problems (aside illegal distribution and the ocassional violent internal dispute "resolution") - that is, no druggies robbing and murdering people, no shootings, no gang initiation rites involving murdering, no crackheads and H addicts dwelling in dystopian street scenes, etc...
Drug decriminalization has been a horrific failure. Legalization just let's these problems become (worse) epidemics.
Taxation in California has proven to be a huge incentive to black market growers who can grow and sell tax free. The legal seller has the weight of taxation.
Your suggestions are basically a recipe for greater disaster.
Legalisation and taxation works quite ok for alcohol and tobacco in most places. And comparatively, it works fantastically better for alcohol than the prohibition the US tried.
What's different for those other drugs? Or should we (re-) criminalise alcohol and tobacco, too?
I agree that decriminalisation is a strange halfway measure. And I can also believe that not all policy in practice is great, even if the bears the label of 'legalisation' or 'decriminalisation'.
"Just after WW2" is an entire lifetime ago. People who were adults just after WW2 are almost all dead.
Japan was entirely controlled by the US after WW2 and had no control over its own affairs for several years. That's one big change. Plus culture changes a lot in a full century.