Google started when the tech sector considered the search problem "solved". You had the likes of Altavista, Yahoo and AskJeeves. There was some variety (eg Yahoo's directory) but no one thought search was going to be a big business.
Then Google came along and ripped them all to shreds from 1998-2006 or so. The most important platform was Windows and Google gained dominance there without paying anyone. Sure, you can argue Microsoft didn't take it seriously (because they didn't).
Then Microsoft did start taking it seriously and the smartphone revolution happened. If you were around at the time, you may remember that many networks simply couldn't handle the anticipated (and actual) Internet traffic from iPhones. IIRC I saw figures that an iPhone user used >8x as much data as the most recent Nokia phone user with Internet connectivity. Why? Because using a browser on an iPhone was a quantum leap forward in terms of power and usability.
Bing was really the only serious threat here simply because Microsoft had deep pockets. So Apple was able to extract (extort) Google to keep it as the default search on iOS.
Bing tried exclusivity deals, most notably with Bing but it was short-lived. Google also pays Firefox but it's less than they earn from that user and no one else can afford that. If no one else can do the same Google wins.
But the main point is that when given a choice, the majority of people choose Google because it is better for most people.
Where exclusivity typically hurts is where it's used to push an inferior product or at least a product where the product isn't preferred.
Now the DoJ could argue that we want to avoid getting to the point where Google has an inferior product but has the market domination and deep pockets to keep out competitors. Maybe that's valid. But I think in the short-to-=medium term, this has simply saved Google billions of dollars a year.
Also, for smaller search engines like DDG, they can never afford the billions Google could Billions was material, even to Apple. What DDG could pay isn't. It's not worth taking.