Google has a say on what goes into AOSP and the builds for Nexus devices. What actually gets shipped on devices is a different matter. The AOSP source is fairly modular so you can actually replace almost anything you want.
Manufacturers try to differentiate and end up filling your device with battery draining crud that makes your phone behave differently to every other Android phone out there.
They then offer updates for a limited time, because porting new builds is difficult as they only got binary blobs from the vendors.
Getting stuck on an old Android build is very bad news because it inherits all the security issues from Linux.
So you either risk running an ancient official build, or put your trust in one of the teenagers pulling together builds from a dozen different githubs with hundreds of updates cherry picked. If cyanogenmod is the only modern build available then sit back and wait for your mobile banking apps to start complaining that your phone is rooted, even though you didn't enable root on it yet.
My next phone will be running windows 10 mobile, if they support their phones as well as they do the desktop.
