Employers and the market will take care of it.
Pot smokers already smoke pot, law or no law.
Some people drive over the speed limit and don’t kill anybody. Does that mean we should abolish speed limits entirely?
There are behaviors that are corrosive and destructive to other people when openly engaged in with no legal restrictions by the population as a whole. That doesn’t mean every time an individual does the behavior, that a disaster instantly happens. But you are still allowed to make a law against it to benefit society as a whole.