It's a chicken-and-egg problem. How can you get web developers to test their websites in Safari, when the majority of Mac devs haven't opened Safari in ages? We have the answer already. Safari doesn't compete on it's own merit.
WebKit is a second-class citizen on the web, and we can argue about why that is, but nobody can sincerely deny it. Apple's strategy has been dead in the water for over a decade. iOS lost.
Safari is still relevant and iOS hasn't lost; the very existence of this thread serves as a clear rejection of that framing. If Safari wasn't competing, or if iOS had lost, nobody would care.
Meanwhile we have people bitching and moaning and meanwhile, as a Firefox user, I go to Safari when I encounter a website that doesn't work in Firefox. Because at the end of the day, developers are forced to test in Safari, and that is a Good Thing. No developer is forced to test in Firefox.