I think alot of people who are really into the specific franchises like 3d Mario, 3d Zelda and Metroid kinda bought a Wii for one of these games, played the hell outta them, felt they werent given more of the same and at some point during second half 2008 /first half of 2009 sold their Wii.
If these games turn out uber amazing and all release this year, I think alot of these people will rebuy the Wii. In that way, you could say these games really are system sellers. Now, especially since the Black Wii is released everywhere. (i'm pretty sure the black wii will relase in the US during this year with a Wii Sports Resort w/motion plus packin, which will be another incentive for these ppl to reinvest in the Wii)
I know ppl (who hated NSMBWii for some ridicoulus reason that I cannot comprehend) whom sold their Wii's and will rebuy it if Galaxy 2 recieves rave reviews.