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.
I'm not sure how much I disagree with you, and your reason is discrepancy in adoption of the add-on as compared to the original platform.
But I will disagree. 3rd parties supported Kinect pretty much just as much as they supported Wii as of 2010 onward. Why is that? It's because MS did a good job at pushing it. It comes back to what I said.
But whether this is all true or not, it doesn't change the fact that it can all be done on one same platform to a certain degree of success or not. The same could not be said for computer components on the 1st consoles of the time, which is what prompted the generations in the first place.