Hype can generate consumer interest and even fool people into buying something that isn't what they think it is, but past the initial launch period for both Kinect and Natal, both will have to sustain on the strength of their respective software/title support.
In other words, people aren't going to keep buying Kinect if games consistently display underwhelming controls or titles that simply aren't compelling to either extended audience consumers (who, contrary to popular belief among some core enthusiasts will NOT buy "any old crap") or core consumers who are interested in more than just party games and mini games.
MS is spending some serious money and effort promoting Kinect, so it's extremely unlikely software support won't be maintained, but it still has the potential to end up as a $150 web cam that allows you to use basic voice commands to watch DVDs.