Earlier quoted context omitted.
The real issue for the Nexus 6 is that Qualcomm has completely abandoned their 32-bit mobile platforms. Look for "8084" here: https://wiki.codeaurora.org/xwiki/bin/QAEP/release - the Snapdragon 805 hasn't received a release since August 2016. So, Google did an in-house bringup for 7.0 - and "normal" OEMs aren't able to do that. Qualcomm usually provides a lot of pre-compiled binaries to the OEM, which either require…
And do they need a new kernel so desperately? I don't understand the problems with backward compatibility. They already provide it via the Android SDK (the support library). Java/Dex/Dalvik/ART or whatever it's called now that runs the shit works on Linux 3.0 probably. (Android 2.3 was released with Linux 2.6.35 [+ patches, I assume && hope], and that's currently the lowest supported API Level (that is api level 9)).
Google should put some resources into kernel development for a year or so.Get the changes they want into Linux 5.0 and make it the android long-term-release kernel. Then no further changes to the kernel unless its 0.0.1 security patches.