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.