In this case we have two ad supported search engines that deliver a very similar experience. As you note the differentiation is not great and thus, defaults matter. So in this case they are no longer competing on product quality, but on distribution. A high barrier to entry ($20B is high) but at <10% of Google's revenue it's affordable to them but basically no one else.
There is also brand competition and Google has done a fantastic job building very strong brand preference. Honestly no one has even tried to build an alternative search brand for over a decade in any meaningful way (DDG and Bing combined is the closest we could probably come and that's like Fresca competing with Coke).
Google Chrome has ~63% market share and that's not up for auction. They pay for ~23% or so at the $20B discussed thus no one is going to be able to out-distribute Google any time soon.
Furthermore no one, including Microsoft, is going to risk $20B on overcoming Google's brand preference. It would take years and probably $100B or more to play this game for an uncertain outcome. Better to invest in OpenAI than take that bet.
Credit to Google; they have built a very strong business with a strong moat. But no, real competition is not a "click away."