In my mind, the only limitation on the sales of Galaxy is Nintendo's ability to manufacture consoles for people to play Galaxy on.
From the day the Wii was launched, I have always felt that one demographic Nintendo can reach is the audience of former NES and SNES owners who will be tipped towards the Wii because of nostalgia and the Virtual Console -- today's retro gamers who miss the days of platformers and turn-based strategy before everything became MMO/RTS and FPS. This market is HUGE and will soak up Galaxy like a sponge. It'll be slow and methodical, like DS sales, where these gamers have also jumped due to the revival of TBS and 2D platformers that moved to Ninty's handheld.
Galaxy will not shift a lot of units in any single week, but I feel it will sell remarkably well every week for several years. Until Wii console supply catches up with demand, nobody has any idea how well any game on the Wii can sell, not Galaxy, not Wii Fit, not anything.
Comparing Halo 3's launch with Galaxy's is an apples/oranges comparison, as most everyone who anticipated Halo 3 already had a 360. Potential Galaxy purchasers are still searching their stores for Wii.
Galaxy's Week 1 sales indicated that 1-in-8 Wii owners bought it in the 5 day window of its release week. In many stores, it was supply constrained. It's continued sales since in comparison to the console have maintained that 1:8 ratio. Now, I've seen some ridiculous Wii lifetime sale predictions: 100M, 150M, and up. I'll be conservative and cut that in half ... say 70-80M Wii consoles sold through its entire lifecycle. That should be enough to push Galaxy to 10M.
