So the assumption is that Apple would run Ads against search?
There is a good chance that if Apple did have a search engine it would be Ad free, which might still depress prices of Google ads, but it would also decrease effectiveness of those ads.
I doubt it. Alphabet generates $175B+ in Ad revenue yearly and Apples always looking for ways to increase stock value. Apple recently introduced ways to scan while depersonalizing your data which implies it would be easier to read and serve ads to users over it. Not to mention that they can afford the development time and have the userbase and consumer loyalty necessary to build and launch a competitor to Google.
Going into the search AD business would significantly hurt one of the iPhone's most significant selling factors, privacy. Because at the end of the day "Apple makes money by selling overpriced hardware, not by selling your data." The moment that changes trust will be much harder to keep.
There is a good chance that if Apple did have a search engine it would be Ad free, which might still depress prices of Google ads, but it would also decrease effectiveness of those ads.