You don't just throw random latest kernel versions at non-PC builds of Linux. The kernel version should be entirely irrelevant to all normal customers, and generally irrelevant to the vast majority of app developers. Why do you think you need an updated kernel? What problem do you expect it to solve?
I am a custom ROM developer.
Latest 4.4 kernel bring massive improvement in performance and security.
More over on future when Nokia 6 goes old due to newer kernel availability we can port newer Android OS and use them