Look at Sega and you'll have your answer.
Nintendo is like Apple. Their software and hardware are nicely integrated and because of that, software development becomes more efficient because they know they're building a game for a specific piece of hardware. Also, they can fully take advantage of the hardware because they aren't having to choose the lowest common denominator in order to take advantage of multiple machines. Using Sega as an example, look what happened to them when they had to go from focusing on one piece of hardware to multiple pieces of hardware that were radically different from one another. Their games suffered because of it. The same thing would happen to Nintendo.
Unlike Apple however where software is made to take advantage of the hardware, the opposite is true for Nintendo. The hardware is designed around the kind of games they wanted to make. The Nintendo 64 controller for example was made to play Super Mario 64. Aside linking hardware and software, there's also a cultural aspect of hardware design. It's part of Nintendo's company culture to build hardware for their games. To take that away would radically alter the company and not in a good way. Again, look at Sega. They were by nature an arcade company. Building their own hardware was their business. It was their culture. Yuji Naka even once said that he was less excited about game development at Sega because it wasn't on their hardware, even though he was also a huge fan of the GameCube.
Sony and Microsoft are different. Their game systems appeal mainly to third parties, companies who always make one game for multiple game systems. They build hardware, treat games as the main feature out of many features, then say "put some games on this thing". They don't really come up with game concepts and then say "let's build hardware that can support this". That's what makes Nintendo unique and it's why the industry still needs them.
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