Using the example above, I know many people that have tried one cigarettes or alcohol who did not go on to other stages. One action did not lead to the other. I also know some people that jumped stages and went to light or heavy drugs without every starting on the more basic stages. Sadly, our world isn't so clearly polar and easy to decide. Despite being logically wrong, slippery slope can still be historically or statistically correct.
Potential consequences matter, likelihood is a valid concern.
Communism doesn't have to become a totalitarian regime that disregards human rights. Marx's theoretical workers paradise is a potential theory and could exist. The whole path is wrong, and you are really using extreme examples of where it is likely to go to show that. I use these examples in many different cases. Well, it is a slippery slope argument, so they are correct to point that out, but it is more than just a slippery slope argument.