And that's why I'll return you to my post about push. If there is no backing behind a peripheral, it becomes an accessory, not an upgrade or an expansion. Expansions work only if they're properly integrated, like for example Brood War, which almost all Starcraft players joined because it was just right.
Also, the point you make on cash-ins may give an answer to the unexpected and possibly bloated success of the Wii, just sayin'. It all boils back to causality. If an accessory has no appeal, then it is not worthy of being a full cycle-enducing piece of tech, let alone warrant a new console.
I'm not sure if you see my point, but it's there.
A software expansion to a popular videogame is substantially different than a hardware add-on to a console ...
Now, there is an argument to be made that third party publishers treated the Wii more like it was an add-on than an actual console, and when one wave of cash-in shovelware was finished they released another, but the Wii actually received steady game releases from third party developers, with the occassional good game, because it was a new system. Had the Wiimote been an add-on to the Gamecube it might have been able to attract 25 to 50 third party games (almost all of which would be awful) in a 2 year period; and after that almost no new games would be released.
If you want hardware to actually be used by developers it has to be shipped with the system.