Addiction is a problem, and it's a problem now, even with the harmful substances being prohibited. The fact that fentanyl and heroin are criminalized doesn't make anyone less likely to end up using them; the criminal penalties just make them even more dangerous to use. You're assuming that legalizing / decriminalizing fent and h possession the addiction situation would get worse -- I'm not convinced that is true.

If we legalized / decriminalized the possession of all drugs including fent and heroin, then people using them would not suffer criminal penalties for being caught with them. Oftentimes it's social factors like incarceration that make it doubly hard for addicts to escape the cycle of addiction. You are caught using and so you enter jail, and catch a felony on your record, making it even more difficult to land work. Or you end up with trauma from being imprisoned, further damaging your mental health, further driving you to escape with your drug of choice.

Long story short, prohibitionism doesn't really work in the sense that criminal penalties don't really deter drug users from using drugs. It just makes it even more dangerous to use the drugs!