[krunkcoin]

"There is already a living example of a custom Apple-designed external graphics card. Apple designed and released Afterburner, a custom "accelerator" card targeted at video editing with the gen 3 Mac Pro in 2019. Afterburner has attributes of the new Apple Silicon design in that it is proprietary to Apple and fanless. [3] It seems implausible Apple created the Afterburner product for a single release without plans to continue to upgrade and extend the product concept using Apple Silicon."

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You don't understand what Afterburner is, and as a result have read far too much significance into it, and haven't noticed that they've already moved on even before the new Mac Pro is out.

Afterburner isn't a "graphics card". It's just a FPGA on a PCIe x16 expansion card. Although in principle you could download an arbitrary bitstream to the FPGA and use it for a really wide variety of purposes, in practice Apple uses it for exactly one function: accelerating ProRes. That's Apple's proprietary low-loss video codec designed for use in video editing software, most notably their own Final Cut package.

They needed Afterburner in the Intel Mac Pro because they didn't want to burn CPU cores on a software ProRes codec, and none of the silicon available in the Intel/AMD/Nvidia ecosystem offers a hardware ProRes codec.

They don't need Afterburner in Apple Silicon Macs because they simply integrated a ProRes codec into M1 Pro/Max/Ultra. I wouldn't be surprised if they were able to share a lot of source code between the two implementations. The M1 version is lots faster than the Afterburner version, which isn't surprising when you know about the penalties FPGAs pay for being reprogrammable in the field.

I suspect the M1 Pro/Max/Ultra project came first, and the Afterburner port was a quick side project. To me, it's not implausible that it's a dead end, instead that's just about all it can be.