Drow and Duergar are born in defended cities. They don't roam around the Underdark like the party in Out of the Abyss.
Yeah but if you lurk in the safety of cities, you won't gain any levels at all. You would stay a level zero npc so to say. There must be opportunity for low level adventure in the Underdark, else there would be no low level adventure groups. You could say of course that the evolutionary pressure on those groups is extraordinarily high, but the chance of survival must be higher than zero in order for this logic to function at all.
Point is, in the Underdark there should be areas that are more dangerous than others and also areas that are more "safe". We dont know where Larian will take us, how long we will be down there and so on. I know we have not that much else to do than talk about the few infos we got, but i for one will save my judgement on this design decision for when i know more.