Why has Bill Gates stepped off now?

Watched a programme last night about Bill Gates and his decision to concentrate on philanthropy in future. I didn't quite buy his body language when he was talking to Fiona Bruce, especially on the topic of the business strategies employed by Microsoft against Netscape, which triggered-off a massively expensive court case brought by the US Government.

What I've read about business moghuls suggests that, however rich, they never want to give up. They can always try to get bigger and outdo, or do in, a business rival. Robert Maxwell's downfall was his obsessive competition with Rupert Murdoch, which got down to the personal. For example, learning that Murdoch had flown to New York for a business dinner at a swanky restaurant, Maxwell immediately got on Concorde and shot across the Atlantic, so he could be at a neighbouring table.

And these people will compete in the smallest way. I read an article which said in passing that while his chauffeur-driven car was waiting at a red light, Maxwell saw next to him a very nice sports car (possibly a Ferrari). He leaned out of his window and helpfully informed the neighbouring driver that his rear tyre was flat, so that as the man glanced back, Maxwell's car could be first through the intersection when the lights changed.

So why is Gates, such a fierce competitor that his employees refer to themselves as "Microserfs", "retiring" at 52? Is it because he is smart enough to know when his business has peaked, and seeing a rival in Google (and a challenge from freeware) that he can't beat (despite his firm's attempt to purchase Yahoo!), he's withdrawn before defeat is clear? In which case, what are the implications for investors in Microsoft?