This explains why nobody buys any Cadillac CTS cars at all?
There are TONS of inefficient vehicles out there that sell great, SUVs, turbo FWD cars (most of which get fairly poor fuel economy), trucks, etc. Most people don't need SUVs or Trucks, but they like buying them, and the ~20mpg or so is acceptable to those people.
G8 is a poor comparison in a lot of ways, mainly due to being released right before the largest economic disaster of the past half-century, and being tied to a brand that was already teetering on the plank. Chevy didn't need the G8 because they were in the midst of trying to fix a lineup with a lot of weak spots, and the CTS fills the RWD Sedan category fairly well.
And you still ignore the Charger, which gets great sales from LE and government agencies, and sells fairly well off the floor as well, despite not being particularly well reviewed or efficient. Ford also has excess capacity to give at the Mustang plant. They could sell regular V6 and V8 variants as Fords, and sell some hopped up / luxo variants to fight the CTS and 3/5 series in the Lincoln stable. It would be a brilliant move.