I think there are multiple reasons behind the desire for scaling monster density for the players who are asking for it. The first one that comes to mind is not the self-serving need to squeak out insane farming efficiency (there are some who seem to definitely want this), but the desire to feel that they are not losing efficiency when farming anything other than Act III. We definitely want people to farm multiple Acts, so we are discussing options to achieve that goal.
Your ideas on the subject are certainly welcome (in fact, the question of "what mob density would you choose?" is a great one to debate) and we hope that you will continue post them.
Act3 already has a good monster density, so it's in no way a self-serving need to be more efficient. We can already do extremely efficient farming in A3, but you're moving the goalpost here trying to frame players wishing to have more variety in the game as selfish offenders. The point is that there's a lot of large open areas without monsters in it. Take the fields of misery, for instance. It's a vast open area, yet it seems completely desolate. You have to run for like 10-20 seconds before finding a monster pack, and when you actually manage to find them, it's a lame 5guys monster pack, or one of those lame treants that barely move. Compare that to the fields of slaughter and tell me which one you're more likely to visit.
And don't just stop at monster density. Monster generation should be randomized as well. Another element that builds onto the lack of variety is the fact that you'll always face the same kind of monsters on the same maps. It would be nice to have different kinds of possible spawns of monster sets every time you visit the same area.