Excellent point. It doesn't take much graphic artist-type talent to make flat design. And that partially explains why skeuomorphism (which is what Susan Kare's designs were) is user-friendly and intuitive whereas flat design is user-unfriendly and unintuitive. It also explains why skeuomorphism is thoughtful whereas flat design is dull. Apple has been using flat design since 2013-present. Apple has used three different variations of flat design: regular flat design, followed by neuomorphism, followed by glassmorphism (aka Liquid Glass).Wiki entry on Susan Kare, from the original Mac team: "Susan Kare is an American artist and graphic designer ... She graduated summa cum laude with a B.A. in art from Mount Holyoke College ... She received an M.A. and a Ph.D. in fine arts from New York University in 1978 ..."
The point is, what macOS needs isn't software engineers but artists. Apple needs people who understand balance, contrast, color, texture and other principles of art and graphic design.
Apple needs to return to skeuomorphism. They can start with rehiring Scott Forstall and bringing back all of the exact same skeuomorpic designs used in iOS 6 and Mac OS X Mountain Lion, but with increased resolution for today's higher-resoultion screens, while adding new iOS 6-style and Mac OS X Mountain Lion-style graphics for everthing that did not exist when when those were released.