Joe Random web developer doesn’t necessarily care whether the browser he personally develops against is disrespectful of someone else’s client-side resources. He just wants to make his website with as little busywork as possible, and testing every website feature for cross-browser compatibility is busywork. If he can avoid the trouble by telling visitors to switch browsers, he will be happy to do so.

This is not hypothetical: developers have been putting up “this site works best (or only works) in X browser” warnings for decades by now.

No, what would happen is that Apple would actually have to spend resources on competing with other browser engines, instead of holding the web back via monopoly. They certainly have the money to do so.