I feel that
not doing this, not trying new, innovative strategies, would be giving up. We already know things that don't work. We know that Regan era "don't do drugs" screens on arcade machines doesn't work. If we were truly giving up we'd be sticking with what we know doesn't work and turning off our brains to the fact that it doesn't work.
Instead what we're seeing is people searching around for new innovative approaches to the problem that haven't be tried yet.
To your point around crime, it is thought that this decriminalization effort may reduce petty theft.
One of the most significant changes being made here in Vancouver is that police can no longer confiscate small amounts of drugs. Up to this point they were doing this very often. Not just drug dealers, but frequently confiscating from regular users.
It is thought that this active policing and confiscation has helped to contribute to a cycle of petty theft, in that the police are effectively robbing drug users and forcing them to spend more of their income on drugs. When a person's drugs they need to feel normal are confiscated, their need for them doesn't go away, and they immediately have a new problem in that need to buy more. Stealing something to sell from a car or store is a very quick way to get that money. Effectively at this point not confiscating drugs is giving drug users more income and reducing pressure to steal to earn more income.
Will people continue to steal for other reasons? Perhaps some. There are also lots of agencies giving free food and there is minimum assistance and (not enough) social housing, so it is argued that the support network is good enough that people shouldn't need to steal to survive.
The next step here as well is that this decriminalization is a path to a safe prescribed supply of drugs, which would further save drug users money.