Causality. You don't know what made Kinect less interesting to 3rd party devs. It could be that it came out after the Wii, it could be that people wanted to hold something in their hands, it could be that Nintendo's marketing push was better, it could be that Nintendo believed in their product more and visited devs about it (like they do for all console launches), it could be absolutely anything.
Also, I recognize the top part of that post ;)
And again, it doesn't remove from the bulk of my point, that it could be done on the same platform, and 3rd party support one of many metrics and you know that.
So name any add-on that third party publishers have given significant support to. Demonstrate that the add-on model has ever worked ...
The NES zapper, Super Scope 6, Sega CD, Sega 32x, N64DD, N64 Memory expansion, Eyetoy, PS-Eye, PS-move, Kinect and Wii MotionPlus have all failed to attract support; what is it about the Wii U tablet that would attract as much attention as an add on as it does as standard hardware for a console?