As I explained before, I believe the chief reason why Apple was even on board with the deal in the first place is because Google is the best, and most people do use Google Search, so it wasn't really requiring them to do anything their user base wouldn't have done anyways.
I don't think the majority of iOS users would take well to having their choice of default search engine shift away from google search overnight. Most would be confused, many would be puzzled as to why their search results seem different or even objectively worse, and you can't make the argument that users don't switch away from defaults, while also expecting disgruntled iOS users to readily change back to Google Search on their own.
I seriously doubt there is any amount Bing or DDG or any other search engine can pay Apple to switch defaults, simply because they aren't Google Search. I am personally on DDG myself, but I also acknowledge I am the minority here.