I think the most tragic thing that could happen for Google is Microsoft paying for Bing to be default search engine on iOS devices. Then users would most likely

I think the most tragic thing that could happen for Google is Microsoft paying for Bing to be default search engine on iOS devices. Then users would most likely realise that Bing and Google are almost the same in terms of search quality and Google's brand would start to weaken. I think the brilliance of Google is their walled garden or family of apps they built around search e.g.(Gmail, Maps, YouTube, Images, Books etc.)