Every time someone buys a computer with Windows pre-installed, MS makes a profit. There is a fairly constant stream of new users, and a fairly constant stream of people buying a new computer because their old one broke down; so how do they increase profits?
1) Come out with a new OS, so people will buy it and install it on their old computer, even though it hasn't worn out yet.
2) Get people so excited about the new OS that they'll buy a new computer that has the new OS pre-installed.
Folks, this is no big secret.