The 'stock Android' label is becoming increasingly meaningless, anyway, unless 'stock' simply means AOSP. Google has gradually been severing more and more pieces of Android (or at least, Google's Android) from AOSP. As each version of Android gets released, more and more processes and APIs get untethered and get linked to either the Play Store or Google Play Services. System UI is next up on the chopping block.
Which makes sense given the horrendous state of fragmentation in the Android landscape (more devices are running Kit Kat today than Marshmallow. Kit Kat is 3 years old) so that way Google can ensure maximum compatibility for the apps that matter, but also it's a way of wielding as much power as possible over the experience by divorcing it from its open source roots.
In that sense the Google Android experience is just another interpretation of AOSP just like Touchwiz, not the 'Vanilla' pure experience we've been calling what ships on Nexus phones. If you want vanilla Android, you'll have to build it yourself and experience the increasingly barren wasteland of AOSP apps.
The genie is out of the bottle so Google can't undo the open source heart of Android, but I'm not sure if that's the path they'd follow if they were to do it again.