One thing that you didn't mention that I think is important is that the GameCube itself was originally supposed to have motion controls:
"Nintendo actually patented and developed a motion controller for the GameCube—Rogue Squadron series developer Factor 5 even experimented with implementing it in their games. Obviously, the controller was scrapped."
''At this time, Nintendo was also beginning to dabble with motion technology. The original idea was to release motion sensing controllers for the Gamecube. However, due to the Gamecube's sagging sales, Nintendo executives feared that if released as a peripheral, the motion controllers would likewise fly under gamer's radar similar to the Link Cable. ''
Third paragraph, before moving on to Wii.