I wouldn't be surprised if the COSMIC folks add it after the 1.0 release. There's been a lot of feature requests for it. In the mean time, a System76 employee created this unofficial extension to let you use other compositors within COSMIC, including Niri.
Don't forget the Apple Mouse with the lightning port under the mouse so you can't use it while it charges. It's still the only Apple product with a design that makes me physically cringe.
I also find it awkward and uncomfortable to use, but that might just be me.
I was never a fan of the skeuomorphic icons (and design in general), personally. I think there's a place for flat design and it isn't all bad, in particular peripheral UI elements, like the top menu bar, dock icons, etc. It is interactive elements that should be the ones to stand out, not everything.
If the homogeny of flat design makes the main interactive elements less salient, then everything designed to look 3D with glossy surfaces and brushed metal outlines does the same, just with noise.
I think there should be a combination of both, which I think is why many find the older interfaces from the 90s and early 2000s much more usable.
That's a long way of saying that I prefer the flat application icons and dock on current macOS. The dock stays in the peripheral and the icons identify the application all while not drawing my attention until I purposefully go to it.
https://github.com/Drakulix/cosmic-ext-extra-sessions