Welcome back to the forums, I hope all is well with you.
There is more than one way to skin a cat.
I think that the "simple black dummies" only truly matter if the characters are of consequence or not. If the characters are of very little or no importance to the plot, or to other characters then generic silhouettes (or otherwise simple characters) would be perfectly acceptable taking very little from the quality of the game.
For argument's sake, there is a subway station established in the game that Sophia has visited before, but never gotten on a subway or shown a ride on one. Subway cars tend to have people on them (Same would be true of a bus, or plane if she needed to ride on one), and usually a little crowd. Suppose she interacts with 1 or 2 people on a subway ride, but 20-30 people are in the subway car. Would it be worth rendering 20-30 completely new and random characters that will have no speaking parts, and just sit there? Of course not, that would be a terrible waste of energy building all of these different throw away characters that are of little relevance to the overall core of the game.
On the other hand, it does improve the quality of the game as you said, but to what degree is the question? I mean twice as many renders would increase the quality of the game too, but that is twice as much work. Making every scene including non-sexual ones into animation would also raise the quality, but again one update every 2 years? no thanks... fine as is at current rate.
I think it would be nice if you took EXISTING characters and put them as background characters as like little hidden easter eggs, and foreshadowing on more wide shots with a number of background characters. For instance, the subway guys... If you didn't have them arrested, or humiliated seeing them in the background hidden off sort of "stalking" at some points but not in great focus would be a nice touch. Perhaps the hobos from the park walking around, or the mail man delivering a paper.
Anyways, back to my original point of there being more than one way to skin a cat. You could save an immense amount of time and resources increasing your production rate if everything was not a scene, and sprites + background scenes were used at certain points to reduce the required number of renders. By rendering every visual as a scene it greatly adds to the workload.