The arbitrary restriction of passenger cabins to class 6 and lower limits our ability to use freighters as large scale, budget people movers. I understand the wish to have the Beluga/Orca/Dolphin as quintessential passenger ships, but they already have the advantage of fitting a cabin type nobody else can use. In a universe where people are willing to basically travel as cargo in Economy pods, why wouldn't any self respecting freighter captain stack them to the ceiling?