Search would sell ad space to whatever company offers the best rate. If it's currently profitable for Google, it still would be, just other ad companies would have the option to offer a better rate.
It was over 10 years ago, but Google did almost exactly this with small experiment Google did (probably related to some suit) where Dealtime (BizRate?) product ads were shown in Google Search results in place of [what was then called] Product Listing Ads.