Microsoft is famous for destroying companies that offer complementary products, either by bundling complementary apps with Windows (Windows Media Player, MSN Messenger, IE) or aggressively competing head-to-head against the most popular ones (Adobe, Intuit). The surviving 3rd party apps are usually ones that are too small for Microsoft to care about. The best (selfish) economic situation for a platform like Windows is lots of tiny complements that have little pricing power but that make the platform itself more valuable.