I'm honestly not sure what you're talking about. Android does have a system update mechanism. You go to settings -> About phone -> System updates.
If manufacturers / carriers change that to check for updates on their own servers, rather than google's - which they can do since Android is open-source, and so they all do - then that's how the system update mechanism will work.
I don't follow what you're suggesting google could do about that, apart from moving more and more OS functions out of the core OS and into google play services. Which is exactly what they've been doing.
Aka, no longer part of Android. The millions of users without Google Play will have an even less functional OS than they used to, all in the name of greater control by Google.