Given that DirectX12 is most likely already a long time into development; isn't it possible they created the custom GPU in the Xbox One so that it also supports DX12?
Seems like a huge fail if they didn't.
Xbox One uses a GPU based on AMD's 7xxx series which are all 11.1, it's not custom and you can't do that anyway, it takes a long time to design a GPU to support the newest version of DX let alone a future version that isn't even anywhere near finished. When the Xbox was in development Microsoft would have been working on DX 11.2 for Win 8.1 anyway. DX 12 would have been in the planning stages. It's not a fail, it's just not a possibility, you just cant do that.
Any future version of DX that has "closer to the metal" developer access would only be something that would benefit PC's anyway. Because developers can already get closer to the metal on the consoles, this has always been the case. AMD created the Mantle graphics API for the PC because DX on PC is a high level API that does not offer low level access right now (not to the same extent as a console anyway) so you always get overhead impacting the performance on PC which makes games run slower. If any future version of DX offers what Mantle does then Mantle will be obsolete and pointless. Of course DX 12 or whatever can offer more features, some of which may or may not work on the DX 11.1 GPU in the X1. But it will always remain a DX 11.1 GPU.