The Mac benefited from all the work Apple has done on the iPhone CPU, but the software scene on the Mac has been deteriorating because developers are using the same tools and the same toolkits to develop desktop applications that are motivated by mobile. The article mentioned here
https://news.ycombinator.com/item?id=42930025
claims to be about the deterioration of desktop interface but it's by someone who's so steeped in the Apple culture that they don't get it is really about the deterioration of Mac interfaces.
Apple doesn't care if you buy a Mac, in fact they'd feel most comfortable if the Mac became a peripheral of the iPhone. It's hard for them to picture developing a new peripheral for the Mac, even something straightforward like the Magic Mouse. A peripheral for the Mac that's groundbreaking? It doesn't fit into their world view.