You can't change the Linux kernel and then close the source and sell it. That's why the GPL is much better for users and other licenses are better for businesses.
Either you believe such a thing is GPL compliant, in which case, yay, they already could do it.
Or you believe they are violators, but nobody has been able to stop them, in which case, that falls into my "in practice, ...".
Because in practice, it has not stopped them
Either way, nothing has changed :)
As for arguably-compliant ways:
Because they can already do like nvidia does, and just have the interfaces be in the kernel, GPL that, and then load binary blobs?
Also remember, even the company doing something more shady (as far as anyone knows), vmware, was not successfully sued for their kernel GPL violation.
So theoretically, they could just drop all pretense and not even do that.
Stating that there are some number of cases of GPL violations that haven't been enforced or are in the gray area is not a logical base for the argument that the idea of having to share the source code should be abandoned. In fact, history shows otherwise - that Linux has had more success (as measured by uptake globally) than any other non-GPL open source kernel ecosystem.
Similarly, just because there may be people who can find loopholes in certain well-intended laws and regulations is not a good reason to abandon their intents. Instead, the questions should be - can we keep these intents and fix the loopholes or make the enforcement more straightforward? I think Google could, but maybe it's not in their immediate interests, one of which may be closer to - how do I upgrade the kernel without recompiling that other stuff.