That's largely another drug war myth. But even if a slight increse in strength is true, we don't see all alcoholics running around with a bottle of 190 proof Everclear in their hands.
You're assuming people buy alcohol for the same reason they buy pot. Not a good assumption. At all.
Look at the marijuana seed ads on the internet. They discuss the level of "high" you get, and how kick-a$$ potent it is.
Many people drink low-proof beer and wine, and very few liquor drinkers go for the 190 proof stuff.