I think it’s not that they stagnated on design. I think they stagnated on caring about customers. That’s why there are so many annoying little flaws all over their products now. Apple at its peak would not tolerate that. But they also have never truly had the ability to listen to customers. Try asking for help in their forums - you’ll get a bunch of fanboys who admonish you for not doing things the Apple way instead of helping you. If that’s the foundation of the company, any lapse in caring about customers will be even more painful.