I'm uncertain about how Scorpio handles compatibility with the Xbox One, and if it won't make some of the same trade-offs as the PS4 Pro.
I thought it was later clarified that the backward compatibility work for Scorpio would not be needed for compatibility with the Xbox One.
Maybe there are differences in how paranoid Microsoft is about binary compatibility, or when the binary instructions are generated and supplied to the GPU, otherwise there are some omissions and incompatibilities between the CI generation and anything after--much less Vega.
The Pro appears to have borrowed some elements from Vega, either in the form of hardware that is relatively abstracted like compression and primitive distribution, or certain instructions that might be shoe-horned in around the existing CI ISA like 2x FP16.
There are instructions that would be dropped or completely reworked after CI, such as various atomics and very different scalar memory instructions. There would also be slight changes within the other parts of the ISA (tweaks to functions, slightly different places in the encoding space for the same functionality) that might mean most of it stays the same as CI, so the Vega-like portions are actually encoded in a way Vega wouldn't recognize.
I think the Pro keeps those parts the same as the original. Scorpio may have a heavier abstraction to allow a more fundamental shift to a different GPU generation. Otherwise, there's either a lot of work to create a dual-compatible Vega, or it has a hybrid implementation that has parts of its architecture rooted in Durango that a next-gen console would discard.
