"In general I'd agree that traditional mGPU support is mostly dying, and given the spotty record of such implementations (in terms of frame time, compatibility, performance) I'm not sure I care."
You're referring to AFR I guess---but true mgpu has been one of the more exciting possibilities of DX12/Vulkan, and if it ever become adopted, most the major issues with past multi card usage would disappear, aka AFR microstutter. Apparently though, it is not as trivial to implement in DX12/Vulkan as I'd hoped, or developers wouldnt need to spend so much time testing it after the original launch, or offering it at all. DX:MD comes to mind---it took them at least a month FWIR to get DX12 mgpu into official release.
I'm sure Nvidia/AMD are in no hurry to see DX12/Vulkan mgpu become widespread, as theoretically it could eat into sales of their high-end cards, being that two 1070s in good non-AFR mpgu would likely beat a single 1080ti/Titan, and be cheaper (in the case of the Titan anyways). So, my guess is that they're not giving devs their best efforts to help implement true mgpu.