I think they may be taking a Linux approach to upgrades which includes "upgrading" kernels but I agree with you that even such an approach has it's own flaws and shortcomings and can only be sustained for a relatively short period of time. Eventually they'll have to come out with a completely new version.[
All of my Win 10 upgrades have gone flawlessly, every single one and supposedly Win 10 is the last "version" Microsucks is putting out, they apparently will be upgrading the OS from here till eternity.
They said the same thing with XP, but technology changes. They will have to change the kernel at some point to address new hardware.
Longhorn is a great kernel, by far the best ever built, by anyone. But it is designed for the Core architecture, it is optimized for that family of Intel computing products. When Intel exhausts the Core line, MS will have to redesign their Kernel.