I appreciate your patience and demeanor,
and hope this sort of discussion isn’t outside the bounds of this forum. Generally people find this stuff interesting, I’ve found, so hopefully it’s OK.
Anyway, I think we’re just talking about a guy’s wall, not his floor, but I’d certainly agree you would not want drywall in a floor. I’d also agree that MLV under a carpet + pad is a great help if overall floor height can’t be increased much. That’s a great point I would also agree that after a house is built, there’s not much use for MLV, however if I were building a new wall, I’d start with 5/8” drywall, and if I needed more mass, I’d just add another layer of 5/8”. More mass for cheap.
Categorically, the great uses for MLV come from it being very thin and / or it being flexible and also being dense. That comes in mighty handy in some circumstances. Another fun fact is that due to it's flexibility it has an extremely high coincidence frequency
. We'll save that for another day.
You bring up yet another good point in that it’s not all about mass. For higher sound isolation you would introduce decoupled framing, make sure there’s a little absorption (cheap fiberglass insulation) in the wall, and look to damp the mass. You quickly reach a point where the continued addition of mass alone becomes a diminishing return.
You mentioned about doubling the mass, which adds 6 STC points an assembly (rough lab rule of thumb). As far as how much sound the doubling of mass will affect, the answer is very frequency-dependent. Doubling the mass could very well stop 99% of frequencies above a certain point, while barely affecting lower frequencies, or any coincidence frequencies.