Kinect was pretty successful during the Wii fad but this isn't some motion controls so not really the same thing. The audience is the same person that plays regular console games. No doubt sales will slow as people who want it buy it but I'm not sure I see the Kinect connection here unless you think handhelds are going to decline in popularity. I personally didn't see the point of the Portal but I'm warming up to it.
Pretty much what you said, but I don't think Kinect even should be compared to Portal.
Kinect needed games designed specifically for Kinect-features. So it failed because it lacked support.
Same as motion-controls for Wii (and Playstation Move).
These were fads and needed dedicated support.
Portal is completely different to them, or typical handelds, because it doesn't require any kind of support other than Remote Play, which is just streaming video-output to a portable device.