Have a look at the results in Portugal (this is a good summary https://en.wikipedia.org/wiki/Drug_policy_of_Portugal#Observations https://en.wikipedia.org/wiki/

Have a look at the results in Portugal (this is a good summary https://en.wikipedia.org/wiki/Drug_policy_of_Portugal#Observations https://en.wikipedia.org/wiki/Drug_policy_of_Portugal#Observ...) - then, come back here and tell us what evidence you've found of a slippery slope. I don't believe you'll be able to find any such evidence, because there is no such slippery slope in practice.