I'll try again.
A significant amount of the prison population are there due to mental health issues such as addiction to drugs and alcohol. You start off with a job and doing some coke on the weekends, drinking every night with your mates, and then suddenly you start getting short of cash but you still need a fix, so you nick some booze from the off-license etc - soon you have drunk the house, no-one likes you any more and you have no job, so you start mugging people, stealing more etc.
Many people in that situation need help, not punishment. Intervention early on to get them off drugs and alcohol, to find them a safe space to live, to improve their sense of self worth and get them back into employment is very successful. Not for everyone, but for many.
By defunding Mental Health services you make it harder for these people to have a chance of recovery and thus they continue to be a drain on the system instead of paying into it through taxes.
And yes, I would decriminalise all drugs. I would make them available at cost price from legal outlets. This allows for engagement with mental health services to try to get people to stop taking drugs, it also completely destroys trade for the drug barons and wipes out a huge criminal network that relies on selling illegal drugs. If it's cheaper and easy to get your fix from Boots than dodgy Dave, dodgy Dave is out of a job as is Mafia Mike.