Prohibition has costs of its own: black markets, organised crime, unsafe products, enforcement expenses and incarceration.
You acknowledge that alcohol has social benefits, but apparently heroin doesnt. Why? Because alcohol facilitates socialisation? Because its culturally accepted? Who gets to determine the "prosocial value" of an activity? Almost any unhealthy behavior can be framed as a societal cost.
Theres an enormous amount of subjectivity here. Some people genuinely enjoy recreational drug use. Others enjoy gambling, despite knowing they will probably lose money.
You are arguing that the state should determine which activities are permissible based on their net social utility. Im arguing that danger alone should not give the state a right to prohibit consenting adults from engaging in any activity. Individual autonomy has value in itself, including the freedom to make decisions that are objectively bad for oneself.