The “too long; didn’t watch” summary is that there’s several peripherals, timers, RTC, etc outside of the processor that software will expect to be present (or at least emulated) with specific interfaces. You can implement x86_64 without a lot of this, or redone in a more modern style, but software such as a Linux kernel will have to be patched for it.
Typically a modern laptop will have all the legacy peripherals emulated, and the modern peripherals will live alongside them.
What's different about modern x86 compared to the old standards?