I’m assuming they are less than that with regards to the advertisement market (I’m not super familiar with the space, but as I understand Facebook is a huge player there as well).
The vast majority of web users have their eyes and attention directly guided by one of these [0]. If they decide you do not exist, then you will effectively cease existing for billions of people on the web because they do not know a web outside of Google/YouTube or the Facebook ecosystem.
[0] https://staltz.com/the-web-began-dying-in-2014-heres-how.htm...
This is the wrong way to think about it. How much would _Google_ be willing to pay to maintain that position? Well, Mozilla's primary source of revenue, is afaik, from having Google as the default search engine, so at least that much, for whatever percent of the browser market-share.
If search wasn't profitable then ads in search and SEO wouldn't exist.
In such a situation you would see a degradation of store product quality vs price. It might not be so clear with internet search because Google’s ascendence was coincident with innovation that it has hoovered up. But once you see competition restored, with things like the return of a Search API, or of more transparent ranking metrics, or of customizable ranking algorithms then you will begin to see what you are missing.