People show up for their salary, they work hard for their bonus. The core part of the revenue is taken for granted as they focus on developing new parts, specifically the ads part. It allows for an up-and-coming exec to say "I made X for Microsoft, you should pay me Y% of X". So even though it would only account for a small % of total revenue it would dominate the thinking. It's how ideas such as ad supported Windows happen despite the long term deterius effect that would have on the core business.