I don’t think assumption is the correct stance, but a healthy skepticism sounds in order.

Alcohol is clearly capable of long term impairment, the debate seems to be about dose response. Nicotine is more questionable, but clearly leaves long term changes in the mind. People abstaining after long term nicotine usage will spend the rest of their lives experiencing cravings. Hell, it seems plausible long term consumption of food types has long term cognitive effects.