Most people don’t change the default option though. If Google didn’t (or couldn’t by law) pay such large sums, someone else (e.g. Bing) would pay a bit less and become the default. Even though most people prefer Google over Bing, there is still a significant incumbent advantage for being the default option.
https://appleinsider.com/articles/17/09/25/apple-switches-fr...
Of course, if and as a new entrant garners increased share, rules should change to reflect that.
The problem is that market dominance tends to be a positive feedback loop, for reasons not associated with actual product superiority. Ultimately it quashes choice, competition, new innovation, and the like.
Current US antitrust legal theory mostly ignores this based on the Bork / Posner / Director (all Chicago School) tradition. See Cory Doctorow's new book Chokepoint Capitalism for a revised take: <https://doctorow.medium.com/what-is-chokepoint-capitalism-b8...>