The problem Natal and Sony's wand face is really developer confidence more than any technical problem. Both of these items are late-generation add-ons being released in the fifth year of the generation, and they’re items which the console manufacturer is mostly using to target gamers that don’t own their console (and already own a competitors system).
Few third party publishers are willing to devote 1/4 the resources to produce high quality games for the Wii which take full advantage of the Wiimote; which will have 80+ Million more users than these add-ons by the time they release. What is the likelihood that they will go out of their way to make a system-selling game for these add-ons?
Most developers will take a wait and see approach to these add-ons, and if they don't begin developing until they have been on the market for a year, and you assume it will take them 2 years to produce a game, it puts the release of this game to 2013 which is (probably) 1 to 2 years after these consoles will all have been replaced.