Bill Gate is just one of hundred people that runs Micro$oft, and having him leaving the post won't have much impact on Micro$oft. Whats more, Micro$oft is already HUGE, HUGE, HUGE company and it is gonna take long long long time to break them down! Resignation of Bill Gate isn't enough to break them down. Whats more, there are hundred, thousands of people who now "Owns" Micro$oft though stockholding. So Micro$oft is gonna to stay.
Whats more, Micro$oft got plenty of divisions and Bill Gates usually don't control every divisions, but more of "Oversees" the operation, so having somebody replacing Bill Gate won't have much impact because it is only "Overseeing" position.
Perfect example: Sam Walton, the owner of Wal-fart... He passed away, did Wal-fart went down with him? OFC not! Wal-fart still grows and thrives.