There is a threshold for the amount of drugs demanded simply because lower prices will not encourage ordinary consumers to demand more. In other words, if the grocery store has a really good sale on candy bars, you might buy some more than usual. Another reason to believe that decriminalization will not have drastic effects is that our society did not fall apart after Prohibition was lifted. Alcohol is far more harmful to society than all illegal drugs combined. There are more than 100,000 alcohol-related deaths a year.
Most campus rapes are alcohol-related.
Because the rights of citizens should not be abridged because the misbehavior of a minority who exercise those rights irresponsibly. If abstract rights don't impress you, consider the practical reasons for eliminating Prohibition. It was a futile effort, a war that could not be won unless Americans completely and willfully forfeited their privacy rights. The government later found that it was more cost effective and morally effective to rehabilitate abusers than imprison them. More importantly, from an analytical perspective, Prohibition inefficiently diverted services from real crime.