Nadella stated that the fear of losing its default search engine status on Apple devices was a significant factor that kept Apple and Google together. If Google lost this status, it could leverage its popular services like Gmail and YouTube to promote Chrome, potentially bypassing Apple's Safari browser.
Microsoft attempted to become Apple's default search engine but failed. They were willing to pay up to $15 billion yearly for this position.
Nadella considered the search engine market to be the most valuable software category, even more so than Windows and Office.