Rather a broad statement. I do know some folks who drink responsibly and a few casual "fatty" smokers who do OK but I know quite a few folks ruined or at a minimum impaired noticably by alcohol and pot use. As for other drugs(I used them all when young except crack), I have yet to see too many folks not have difficulties with heroin or crack or cocaine or crank or X or LSD. I know there are exceptions but amongst habitual users it usually becomes problematic and not only because it's illegal. I came of age in the early 70s (the nadir of drug use...and acceptabilty) and everyone I know who moved on to a productive life totally gave up hard drugs and minimized alcohol and pot use. Just my experiences and observations.
I would agree, and the result is the same whether or not drugs are legal.
IMO The difference between legal and illegal drugs is that legitimate businesses don't typically kill their competitors.