The changes in both the mobile and desktop operating systems signifies which design style currently works best. Until we get morphing displays, soft UI is here to stay.
The initial iPhone OS had rather square looking toggles and panels. But we all know that our primal conditioning still has sharp corners paradigm as “teeth”. That makes them more frightening and less friendly.
The layering uses the same well-known concept of having a lighter surface closer to the user, while subsequent, lower levels of the stack get a little bit darker. That shows hierarchy even without the translucent effect.
The Mac OS toolbar icons are also becoming more uniform with typical iOS / iPad OS icons. The icon style is once again rounded, friendly and open. The strokes are thick enough to be easy to understand and icons don’t have any unnecessary decorations.
Previous Mac OS versions also brought Dark Mode and San Francisco (font) from the mobile OS. Now Apple Design system is very close to being consistent across all their platforms.
San Francisco is a modern, sans-serif typeface that proves once again that simple is great.
This whole redesign also proves, that a company with millions of users is likely pushing the UI in the general direction that works.
Sure, a product designed to work for “all humans” will need to follow the most common-ground friendly guidelines. That doesn’t mean we should use those same rules in everything we do.
If you’re making a highly specialised, niche product, you can still use sharp corners, fully opaque layers and so on.
But I don’t believe Apple didn’t do their homework. They surely did extensive research on what most people like, and it happens to be the trend that’s been gaining momentum for a while now: Modern, Soft-UI.