Apple are paid $15 billion annually by Google for the Safari default. That's around 10-15% of their profit for doing nothing other than setting a default.
I've always wondered about the lost revenue possibilities of Apple not building their own search engine. Imagine in a perfect world (for Apple) that all Apple devices used AppleSearch 100% of the time. What kind of revenue would they need to pull in to clear 15 billion in profit per year?
Some back of the napkin math shows that Alphabet is averaging about a 30% profit margin the last few years. Of course they do a ton of other stuff, most loosing money, but let's say Apple can make 30% off their search engine. That'd be 50 billion of revenue per year that Apple would need to bring in to break even with Google's 15 billion payment. Seeing as they already pull in ~20 billion per year revenue from App Store ads alone, I think it's not far fetched to think that Apple is leaving money on the table by not rolling their own search engine.
I think they must be working on something, even if it's just as a backup plan in case Google is prevented from making such deals in the future due to anti-trust issues.
Huge problem for Google who will lose the most lucrative and dependable revenue source they have.