Works really well for Google and Android where they offload handpicked versions of the Linux kernel to the phone vendors to maintain. In practice they don't bother with it so phones never get any updates after 1 year.
Maintaining custom Linux kernel fork is not possible if you want to keep it up with the upstream changes.
I don't understand what you mean here. It's my interpretation that keeping up with upstream is what was meant by answering to the community. It seems that the GP post was not concerned with this or with following the footsteps of Android. It certainly seems within Microsoft's means to deploy a custom kernel on Azure and then only periodically merge with upstream (I believe they are already doing this anyway).
It'd be much better if they upstreamed the changes instead of fragmenting the entire ecosystem with custom forks.