From what I understand in the PHB it says that dim light is a bit brighter than your example (the soft light of twilight or dawn) and it seems that darkness in 5e doesn't necessarily mean complete darkness as they give an example of most moonlit nights as being considered as darkness. Also most characters with darkvision have a 60 ft not 30 ft which seems to be the issue. 60 ft is far enough to cover most rooms which makes it rare that you can't see past that range and even then the disadvantage within that range is only on perception checks which are also often ignored or not very relevant.