Too many things wrong with these comparisons. Let's look at ShadowFall - it's poised to overtake KZ3 - and launched on a FAR lower userbase. No new Halo has released, so that's irrelevant. The "brilliance of Uncharted" and what's in your collection are also moot. Wii Sports - um, nobody suggested that Uncharted 4 would sell like it (not sure how such a comparison even makes sense when speaking even figuratively).
Bottom line, install bases grow with new releases, especially from acclaimed series'. Thus, saying it will sell 6 million max is low balling the game.
Ok. I will buy it day one, if i am alive and everything... =D