I suppose one may presume more people will get buzzed if it's legal, but how many people reduce or eliminate other things? Probably not zero. Statistics from legalized states don't seem to support negative societal consequences, that I've seen.
I do think that if the price of the legal stuff is made too high through taxation, then there's a lot left on the table, because getting rid of the black market and associated negative consequences of that could be make it a bigger win.
As to impairment while driving, there is lots of data showing alcohol is very dangerous. Being sleepy is actually more dangerous (causes many more deaths than booze). I wonder just how dangerous a THC high is. My experience is over 40 years old, but it seemed like THC influence made people go slower, and they seemed more attentive (as opposed to alcohol where it was faster and less attentive). Not that I want a bunch of buzzed drivers out on the road, but a knee-jerk suggestion that all impairments are equal isn't logical.
I do kind of worry about that certain phenotype who might get into a habit that drains traditional "drive", but I've seen others who manage it well, and in fact thrive with it. That example would be an artist friend of a friend who makes a good living, producing her work while under the influence.
It's going to be used whether legal or not. If I were king of the world, I certainly wouldn't try to stop it.