Adsense controls like 80% of ad market. Biggest thread to Google are walled gardens like Facebook or Amazon. Search engines like Bing and Duck are not even on their map.
> dominance ... which is based on the quality of the search results
Any source for that? Google search already provides inferior results in many areas, but it is still dominant. Google also pays billions to Apple and Mozilla to be default search engine.
And if threat emerges and gets like 30% of search market share, Google just dials down ads and make their search experience a bit less crappy.