It's quite possible that DOJ antitrust action would prevent Google from paying to be a preferred search engine in the future. Once Google isn't paying you, why not provide your own search engine (and cut Google off from that valuable data stream)?
My prediction is that they will try to monetise this somehow. And the most likely way to do this is via some privacy friendly advertising model (or an ad/subscription hybrid). They're already doing it with Apple News.
I'm unsure as to whether they would seriously create a new ad platform, but a key tenant of the company's culture is not relying on advertising for revenue.
BTW their maps revenue from Google was a bigger share of total revenue when they gave it up than their search revenue is now.
Apple made over a quarter trillion dollars in profits in 2019 (https://www.statista.com/statistics/267728/apples-net-income...), and gets somewhere around $10B from Google. (https://www.cnbc.com/2020/10/21/apple-services-success-story...)
They can certainly afford to ditch the partnership.
I could see them coming up with some sort of privacy friendly ad targeting system using on-device matching or differential privacy.
So they solve the privacy problem but there will still be ads (unless you buy their "Apple Ad Free Experience" service for $10 per month)