My annectodical experience is, that lifelong heavy drinkers who drink the same as a heavy smoker, are long dead, before reaching the same brain damage as pot smokers.

Brain damage from pot is mostly reversible, though. Damage from alcohol less so.

In either case, Dosis sola facit venenum (only the dose makes the poison).

So a person who used to be a heavy drinker before, would be better off, with a occasional smoke. A heavy pot smoker, with the occasional beer. And the wors of both worlds, is to combine both in excess.