It's not about "sales targets," it's about Apple releasing a new, cheap OSX every year and people salivating. The hype goes into overdrive, there are a billion articles covering new features, a billion articles reviewing it, and people generally flocking to Apple stores to punch each other in the face in order to buy it first.
While Microsoft will never have the rabid fans that Apple does, they can certainly create a new version of Windows event, much like those Apple events, to keep people talking and interested in a way that's difficult when you only release every 3-5 years.