This part is why I hope x86 has a long life ahead of it. Everybody and their dog have their own standard for things in ARM-land.
Most manufacturers take standardized IP and IP-providers provide an appropriate linux drivers. Few manufacturers are willing to develop and maintain their own GIC hardware and GIC driver, most of the time they just take the ARM standard GIC.
The fact that Apple does not provide drivers is a consequence of the Apple business model. And the fact that the ARM ISA does not stipulate a unique GIC is actually a strength. It makes the architecture more versatile and suitable for evolution (maybe Apple found out that the ARM standard GIC is not complete enough for them).
We have plenty of examples in the wild. Just look at the state of Pine64 and u-boot, for example. It's a mess of standards.
And what you see as a strength others don't.
It seems pretty standard to me, not a custom GIC as Apple.
And yes, in u-boot there are plenty of device-trees for each target. What's wrong with that?
The device tree is usually provided by the manufacturer, the compiled device tree is usually very small and allows genericity.
Despite the benefits ARM provides amd64 undeniably has advantages and non-trivial ones at that.
personally, I'm of the opinion it'll come to a coexistence for a time rather than one dominating the other immediately.
Anyway, both amd64 and Aarch64 are already being used outside of Macs/Apple products.