Take a look at this speach by Greg Kroah-Hartmann, THE kernel maintainer. https://youtu.be/RKadXpQLmPU?t=19m20s

All big Android device manufacturers are painting themselves into a corner. They are the ones that have HUGE problems with support and longevity. Software maintenance wise the devices are dead bricks when out of the factory. Replacing 88% of the mainline kernel with their own crap is f***ng crazy. It is unbelievable how stupid it is. People are walking around with huge untested crappy software blobs in their pockets.

Fortunately Greg has been talking to the big SoC providers and they know they have a big problem and are trying to work on it. And Google is now demanding that device manufacturers use the new LTS kernel versions.

Purism will use a mainline kernel and will avoid all of those problems.