I agree that OpenAI could and most likely will execute quite well on ads.

What I'm uncertain about is how much the ability of Google to set defaults matters.

Setting Gemini as the "AI" on phones, automatically integrated with all "daily" services could matter a lot. They have a platform ready to go and are pushing hard to make themselves really attractive. All while being very profitable.

Apple on the other hand will be in a strong position to negotiate a good deal with competitors to OAI and my suspicion is that "good enough AI" is all most people need.

And of course there is the financial reality that OpenAI does not only need profits, but profits on an enormous scale. Just being successful would mean they missed the mark.

My personal guess is that Microsoft will fully buy them at some point in the future but I'm not, confidence enough to bet any money on it.