If you go beyond the Gamecube then they have to update the graphics
Care to explain why exactly Nintendo would HAVE to update the graphics? It seems to me Nintendo isn't concerned about graphical updates, because look at how a lot of their core franchises looks nearly exactly the same as they did from the N64 era (Animal Crossing, for example). And look at the Miis and how those look like they were from the N64 era as well.
I don't know how anyone could claim with a straight face that graphics are important to Nintendo, so I see no reason why they would be compelled to update them. Every bit of evidence we have to go on shows that Nintendo is perfectly content to leave graphics as they are and just take on wiggle-waggle control or some other minor features instead.
So the question is, with graphics kept exactly as they are, how much would it cost to tack on motion control for these older games?