Amazon's Fire OS is android but they've announced a new webOS ripoff, VegaOS, Linux running HTML5/react apps. Normally I'd think this kind of diversity is a good thing, but apparently it's locked down even tighter and side loading into Vega OS just isn't a thing at all.

Edit: the point I was trying to make is that some vendors are working to get away from Google's Android foundations completely. I would think that graphene could go that route if the rug was pulled. But good luck with device support when the device specific source isn't released.