Especially since Bing's contracts have historically required disabling the ability to switch to a different search engine, presumably because they realize that

Especially since Bing's contracts have historically required disabling the ability to switch to a different search engine, presumably because they realize that everyone would just switch to Google anyway, meaning that it wouldn't just be the default but the only search engine people could easily use unless they jumped ship to Chrome. The problem isn't that Google is the only company that can pay due to their monopolistic position, it's that they're the only company willing to pay that wouldn't require Mozilla to compromise their key principles and their browser - probably because they're good enough that they're confident people actually want to use them.