Registry backups shouldn't be expected to be any easier with a different kernel. In fact, if anything it will introduce a lot more bugs, even just from programs that rely on kernel bugs.
There's no reason to believe NT is any less 'fresh' than Linux. It undoubtedly does some things better. Even as a Linux user, throwing that away would seem a massive shame.
While basing Windows on Linux may make some development easier, it also throws away the unique proposition that Windows provides.
It would take a lot of internal motivation to overcome the people who founded their careers on building an operating system for Microsoft.