If you believe the only reason people are using and developing for Safari is because of Apple's monopoly, then that's a real cost or harm to users and developers isn't it? Compared to a theoretical or possible cost of a Chrome monopoly that may not happen. And if it does, then that's what antitrust is for.

I don't want to give Apple a monopoly to protect me from a different monopoly. I want to give power to antitrust to protect me from all monopolies, include Apple's and Google's.