> Antitrust rules which prevented anyone from bidding it would protect against this.

why would anti-trust rules prevent _anyone_ from bidding? Apple can sell their browser search, just like mozilla can sell firefox search. And anyone with a browser could do the same. Unless the anti-trust rules somehow become so overarching that the selling of space for advertising becomes illegal?

You highlight some genuine points of difficulty for antitrust enforcers.

If the rules were targeted at Google only then Google's lawyers would argue this is unequal application of the law. Even if the courts rejected Google's argument there'd be a real risk end up with exactly the same situation but with Bing in a couple of years time as they become the default search on every device / browser.

If "pay for default" deals were banned altogether then Firefox might be seriously hurt, which isn't exactly good for the competitive tech ecosystem.

I think it would be a good move to prevent browser deals. There is no reality in which the winner is Firefox, Kagi or DDG - it will always be Google or Bing. That's clearly anticompetitive - it locks the other browsers out of a major share of the market.