I think its more that the core concepts of a desktop UI were correct 20 years ago and other than minor refinements, there is no reason to change it up just for

I think its more that the core concepts of a desktop UI were correct 20 years ago and other than minor refinements, there is no reason to change it up just for the sake of change. If you look at apples other products (iPhone, iPad, Apple Watch) they all have radically different UIs which suit their form factor best. We see this in basically all established industries. The bicycle of 30 years ago looked pretty similar to one you would buy today. Its just every part has been made slightly stiffer, lighter, more durable, more vibration absorbing, etc but the core shape is still the same because we already came to the correct design.