@Zeljedi and Squlliam
There is nothing saying that the either company will go out of business immediately because of their moves to copy the company that disrupted the market, but over the next 5 to 20 years their attempts to attract gamers outside of the demographic they control can destroy the companies existing fanbase to a level where they may not recover.
There are (probably) many ways that this could happen, but the most likely situation at the moment would be that Sony and Microsoft release their add-ons and only really develop competition for the simplest and most gimmicky games that the Wii currently has. Being that the Wii's userbase is so large, for every one of these games Sony or Microsoft release the Wii will see several similar titles which are (generally speaking) higher quality in the same timeframe. The result of this is that Microsoft and Sony would be seen among these gamers in a very negative light heading into the next generation.
At the same time, since Nintendo made their control method the focus of the console every game is designed around it, and the steadily increasing third party "Core" games support starts to find ways to use the Wiimote (and Wii MotionPlus) in ways that greatly increase the quality of the game and can't be replicated on a conventional controller. Over the next few years, the Wii gains "Cred" with the "Core" gamers and maintains most of its support from the mainstream and expanded market.
Now, since Sony and Microsoft release these add-ons to their system late in the generation they believe that the momentium from these systems will prevent them from having to release a new system in the near future; and since the systems still have good graphics then there is little motivation for people to buy newer hardware. Nintendo takes the opportunity in 2012 and releases new hardware which greatly increases the quality of their overall control method and makes a (very) noticeable improvement on all the technical features of the PS3 and XBox 360 while retaining a very affordable price of $300 while bundled with a game.
Much like the release of the PS2, their next system quickly kills of both conventional and expanded interest in their competitions products and they rapdily build a large userbase. By the time their competition ramps up to release their next console there could be 30 to 60 Million systems already on the market, and even after their competition releases their systems their total combined sales is below that of the next-Wii which means that they are falling further and further behind.
I don't know if this is going to happen, but the decisions made by the companies do not seem unrealistic, and (in the past) similar things have happened. When you compare it against the SNES the N64 didn't do that poorly, and the Gamecube seemed to be a step (or two) towards correcting all the problems the market had with the N64, and yet the disruptive nature of the Playstation and the earler release of the PS2 meant that the generation was (basically) over before the Gamecube could launch.
