In my opinion New Super Mario Bros Wii seems to be targeted at the new audience and not serious gamers, (thats why the super guide is there, which inherately is kind of pointless as a feature for us gamers, but for a non-gamer its probably quite handy).
The Mario brand is very strong, and Nintendo probably have already cooked up a new Mario game, (or at least have the initial ideas for it), but are not going to let anyone know about it till they increase their fanbase, (think of how fundementally similar Mario bros 2 (the japanese version) and Super Mario World were to their direct predecessors).
Galaxy 2 however may be suprisingly different, look at what happened with the transisition between Super Mario World and Yoshi's Island. In this day and age a change like that wouldn't really work, time and money are much tighter these days, and the game could end up becoming such an organisational mess, it will be delayed, and then it will be made out of date, (think of the "Duke Nukem Forever" effect!).
I personally don't mind Valve's decision on Left 4 Dead 2, I'd rather have a few slices of toast when I'm hungry, than wait for the cake and starve, (bad joke I know, but you can see what I mean!).