Sure, treating a GPU as a SIMD blackbox may work for many problems as a suitable abstraction, but in doing so you also overlook many of its capabilities. x86-64 can emulate many of the SIMD aspects without too much trouble, but the aspects like huge number of processors with many registers is not something that is able to be reproduced without a large number of tradeoffs.
The only way that I see it as being possible to have a true CPU/GPU hybrid that is effective would be to basically have two separate chips for the GPU and the CPU, maybe multiple chips. I think the reason why such a product has not really taken off is because at that point there really isn't a point over using it versus a separate GPU and CPU. Maybe if hardware designers figured out how to greatly improve CPU to GPU communication in such a setup over having the motherboard in-between it might be worth it. CPU to GPU communication is a bottleneck for many applications.
But in the late 80s and early 90s they actually had the i860 and its variants: https://en.wikipedia.org/wiki/Intel_i860
which contained a graphics processor! That wasn't a particular success but it landed us the MMX ISA on x86.
Surprisingly enough, Intel went then to make similar mistakes with Itanium, and then with Larabee. But both things landed us features on x86.
Maybe Intel ~1998 could have solo-launched a new architecture, but the only way they'd get uptake now is something in cooperation with AMD.
And maybe the AMD partnership bridge is burnt from previous shenanigans, but it seems like both AMD and Intel would have incentive in more tightly coupling graphics compatibility to CPU ISA, vs Nvidia designing their own.
That said, in that hypothetical reality, Nvidia wouldn't have been able to innovate and execute nearly as fast as they have.
As one of my Comp-E professors once quipped, "If a structural engineer ever tells you programming close to processors is easy, ask them how they'd like their job if the physical properties of lumber changed every 2 years."
Also, the game they were using to validate the performance of their hardware (Doom, I think?) ended up being so different from other software in how it used the GPU that their optimizations didn't really transfer.
I suppose some of the "DNA" lives on in AVX-512..
I think there is that latency vs bandwidth trade-off where CPU likes lower latency and GPU higher bandwidth, but you can't achieve the same with a single chip.
Naive question, because I literally don't know the link between CPU instruction stream and GPGPU instruction stream.
But it seems like there would be an opportunity to seize the higher (open) ground at the ISA level, and then force Nvidia to implement its own support for that standard.
With the point of being able to run identical code across CPU / CPU-with-embedded-GPU / CPU + GPU.
Understand we're talking about mind-boggling levels of complexity here, but it feels like the CPU shops ceded the role of graphical ISA to Nvidia & Microsoft (DirectX).