Even if DuckDuckGo and Bing pay absolutely nothing, it's good leverage to make Google pay even more, and its already public knowledge Google pays a lot to remain the default search engine on iOS, to the tune of billions of dollars a year:

> https://9to5mac.com/2021/08/25/analysts-google-to-pay-apple-...

> https://www.npr.org/2020/10/22/926290942/google-paid-apple-b...

etc, etc. Given Google pay a lot more for this privilege today than in years past, this may well have been part of the strategy.