I expect most DDG users would stop using it. Apple may be the "least bad" of the big players when it comes to privacy, but I have no trust in them and no interest in using their services.

It would probably make a lot of sense for Apple though, and I'm sure they'd convince more Apple loyalists to switch to DDG from Google by putting their name on it.

Then Apple would have to give up the truckloads of money they get from Google for making them the default search engine on Safari.