CPU emulation isn't the biggest problem for emulating most platforms. It is the GPU that is usually the biggest problems. Yes, Xbox and Xbone both use x86 compatible CPUs, but the Xbox used an nVidia GPU and the XBone uses an AMD.
The limited backwards compatibility of Xbox games on 360 was achieved by rewriting some of the shaders. nVidia threatened to sue Microsoft, but didn't because Microsoft didn't try to emulate the nVidia chip.