I don't think that the US lacks harsh punishment at all - compared to peer countries, prison conditions are terrible, the incarcerated population is massive, etc.
What I do agree with is that enforcement feels ineffective. Every time a criminal commits an offence, they are spinning a roulette wheel and hoping it doesn't land on "go to prison". Making sentences longer and conditions worse doesn't change the odds that a particular criminal will end up punished, it just means the downside is worse (and increases prison populations). Even when a criminal is caught, they're often bailed and able to abscond. The question should be how to increase the effectiveness of enforcement whilst preserving rights and protections against wrongful punishment.
The other side of the equation is looking at the carrot, not just the stick. If life sucks and people turn to criminality because they feel like they don't live in a society that gives a damn, you have to improve society so people have something to live for.
I think a lot of this weirdly needs to be "vibes based policy" as it were. People need to believe that justice will be done in order to feel secure, and to feel that if they were to commit a crime that they wouldn't get away with it. Objective reality doesn't control behaviour, peoples' beliefs do.