Isn't, at all. Something not being easily available makes it more attractive and makes people more curious about it. Such is human psychology.

Prohibition of anything for which there's demand does not work. It can't work. In general, laws only work when the majority of the society is on board with them. If the only people who are willing to enforce a law are police officers, then such a law only works when there's a police officer nearby.

> Alcohol is legal, how does it follow that there should be more iterations of addictive substances that do more damage to the fabric of society?

Again, if there's demand, there will be supply, regardless of legality. And since there will be supply of addictive substances either way, it makes sense to legalize them. The legalization will remove the criminal element from the cost, making it significantly cheaper, and would also ensure high quality/purity. I see no downsides here.