Microsoft and Qualcomm together supposedly just invested tons of money in making sure that x86 emulates well on ARM64, but apparently had little success emulating x64 on ARM64. If the speculation and rumors about an ARM transition have merit, it seems backwards to sunset x86 and not x64.

Which it seems unlikely that Apple could do in secrecy what Microsoft + Qualcomm could not do (very publicly), but I guess all things are possible.