For example, let's say I could be curious about DuckDuckGo if I had to choose a default search engine when I got an Apple device instead of Google being the default, then that could be revenue to DuckDuckGo for them to improve their search engine.
Not really sure what the DOJ expect Apple or Google to do in this situation, it seems to me that 2 companies entered a mutually benefiting contract. You can't argue that Google "colluded" with Apple, there's a lot of throwing punches between each other (all the privacy oriented moves Apple is doing are hurting Google's business) and again Apple could be making its own search engine anytime they wanted, they already replaced Google Maps with their own thing.
If the only two players (Google and Apple) both use Google Search by default, then Google has effectively captured 100% of the mobile ad market. (And since Google is paying Apple et al for that default state, it is indeed a market.)
The court could require device makers (including Google itself) to prompt users to select their search engine provider, or potentially ban Google from buying search engine defaults from other companies (Apple, Mozilla, etc.) as an anticompetitive practice.
Sure, they could sell it to someone else, but without the biggest player bidding up the price, it'd probably sell for a fraction of what it does today -- which could be devastating to Mozilla, given that almost all of their funding comes from the search deal. What would that do to competition in the browser space?
Using Firefox as a reason not to break up some of Google's hold on search seems extremely short-sighted. Besides, Edge using Chrome changed the whole browser market dynamic as more and more people are using it. Once Microsoft reaches a somewhat decent percentage of install we will be back at a two (three?) player situation.
Yahoo has been Bing for years, and DuckDuckGo pulls from Bing and Yandex - the latter being mostly for searches in Russian.