Default presets are not the user "choosing" something. It's why companies always choose the opt-out method vs opt-in.
That's been my experience and based on that I can see it being justifiable for Apple (who is after all a very opinionated company) taking a similar stand: we don't like defaulting to Google but they're just the best game in town so if we want the iPhone to be the best phone out of the box it needs to default to Google.
The big issue is Google's monopoly on english-language search, which together with Youtube gives it massive amounts of data on every person in the world, as well as giving it immense influence over public thought. I can see Youtube's monopoly ending (certainly, Instagram could have done it had Zuck not killed Instagram's attempts at long-form videos), but the gap between Google search and other competitors (only realistic ones are Bing and DDG) is immense, and probably insurmountable.
Apple has a higher market cap than Alphabet, Inc.
Apple makes so much money that they’ve given back more than 200B in buy backs and billions more in dividends, Google has yet to return anything.
Many people may not event know they can change that setting if it is not proposed the first time.
It's gotten much better with the recent ability for one iOS device to prime the pump for a new one, but I have a hard time picturing Apple treating this setting as vital to the device setup.