DDG's ads wouldn't need to be as profitable as Google's, they'd only need to be as profitable as what Google pays Apple to use Google as the default search engine, which is likely less than what Google makes on ads on searches from Macs.

Would they need to make a return directly? If they could buy it and put significant resources on it, they'll have a direct competitor against Google, with the advantages of coming by default on Apple devices and of being privacy friendly. Just that might scare Google to pay them more to leave the Search space and put back Google as their default search engine.

In 2014 I thought that Google paid Apple around 1/10 of Google's search ad revenue from iPhones, but that number involved quite a bit of guessing.

They could do what they usually do, and make the service available only to purchasers of Apple hardware.