It seems to me Mac users care about using native UI toolkits more so than Linux or even Windows users. I would suppose that has to do with how polished and integrated that platform is.
As a Linux user I really don't care if you use GTK for the UI. I've long given up on customizing the look and feel of my system to the point that application integration matter.
For many decades, there have existed commercial apps native to the Apple ecosystem, often from small/mid-sized studios, which would use native UI to signal their premium craftsmanship. I think that for many long-time Mac users, seamless/native UI is still a signal of quality in third-party software. However, this sentiment is moribund. That entire market is now dwarfed by the ecosystem of “cross-platform apps” (formerly Windows ports, now usually web apps) and Unixy OSS tools; all the while the Mac App Store now peddles lots of native-UI apps of deeply questionable quality.
long ago, the integrated feeling of Apple MacOS was a great selling point; decades later, daily use of the web, more phone than desktop, Windows low-bar junk and other factors change the equation.
If a smart user here today says that recent network-centric apps are OK with him visually, then maybe they are OK?
I think Apple made a strategic error when they failed to embrace DOM/browser engine-based toolkits like Electron on the Mac. If they had applied their leverage, of which they have plenty, the technical challenges of better native integration and support for the wide range of traditional Mac affordances would seem to have been quite manageable.
There's some reason to your proposition. Directly integrating/"embracing" foreign toolkits/web engines is the last thing Apple would ever do, but exposing more of the underlying machinery to allow third parties to integrate better - yeah, sounds good.
On the topic of Rust GUIs on Mac, this was recently in the news:
As a Linux user I really don't care if you use GTK for the UI. I've long given up on customizing the look and feel of my system to the point that application integration matter.