Why is popularity a justification for legalization of deadly addictive drugs?
Alcohol and Tobbacco are not even in the same league as hard drugs such as MDMA.
Nonsense. Alcohol is highly addictive, and tobacco is #1---more addictive than heroin or cocaine. And alcohol can kill you in a single evening of use.
the only positive aspect to legalizing all drugs would be elimination of the black markets for these substances.
"Only"? That's a HUGE benefit. Think of what the black market involves: innocents killed in drug-turf wars; user deaths due to contaminants and unexpectedly high purities; inflated prices that motivate crimes by users (whereas winos can get their next fix by collecting cans or panhandling); enriched criminals; and corruption of the justice system by criminals' riches.
There's no easy solution to the drug problem
The only problem not solved by legalization is the self-destruction of some users---but there is NO governmentally imposeable solution to that problem.