I am hoping that we seriously evolve the ATX standard to allow for a socketed GPU board that can also enable user replaceable memory. Seeing an enormous GPU that is larger than the motherboard itself hanging from a PCI slot feels like horse and buggy shit. I'm imaging two boards back-to-back connected by a central high bandwidth bus (which could also do power delivery) that would allow one side of the case to be for CPU/RAM and the other side to be for GPU/VRAM.
Your solution only allows for one GPU, maybe two if the motherboard is really huge, and it doesn't really solve the slotted VRAM problem.
PCI is (was) allowed to be even longer. Old AT and ATX cases had a slotted support bracket to hold the far end of the PCI cards. See how an Adaptec 2400A looks like.