Android changed Linux kernels with every single release, from 2.6.27 to 4.10 by core release, and vendors often update kernels incrementally in between. Any driver issues are trivial to change presuming they didn't come in a binary blob and the source refused any further changes, which is supposedly the case with Qualcomm hardware in the Nexus 6, for instance.
"Android has been trying to fix this with treble[0]"
Treble is about the intra-Android interfaces, not Linux device interfaces. This is about Google doing large scale, breaking changes that made vendors just throw up their hands.
"Another large problem is that Linux was designed for desktop architectures"
Nope.
The reality is that everyone always thinks they can rebuild it better. Second (Third, Fourth, etc) system effect. It the very basis of NIH syndrome. So some small group at Google wants to research OS'. Whatever. The likelihood that this replaces Android, or that it is in anyone's imaginations that it actually will besides content writers outside of Google, seems very low.
This is why all of the Google's code sits in one big repo (reportedly). It allows them to refactor all dependent code at once when changing interfaces.
Android has been trying to fix this with treble[0], which will make many more phones get OS updates going forward. So Android has attempted to work around one of linux major problems.
Another large problem is that Linux was designed for desktop architectures. It's design and focus continues to go that way. This has impact across an entire range of functionality that an OS is supposed to bring.
Fuschia seems to be a research project with a focus on mobile/laptop type devices. It could care less about server type environments, which lets it make different tradeoffs in design.
[0] https://source.android.com/devices/architecture/