They could for a while but what's the point if it's not sustainable? Google is not going to turn around and make it more open again.
replyWith every Android release you will build up more feature base to replicate. Unless you cut all ties and drive a separate ecosystem but good luck getting enough developers to buy into that.
Well, someone is clearly giving Graphene money, and quite a lot of it, as per TFA:
reply> We recently hired a bunch of new people and will be hiring more so our progress will be accelerating.
And the US might grow a government with a spine and take Android away from Google. It's all hypothetical.
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