Just want to point out that not all 16-bit code can run unhindered in long mode. You can run 16-bit protected mode code. But long mode dropped the V8086 mode that made easily emulating 16-bit real mode possible.
But you can switch in and out of long mode, so it is also possible to basically do whatever you want. And you also have hardware virtualization. It is a political decision to drop compatibility for 16 bit Windows and DOS (i.e. it was too much programming effort), don't blame the hardware.
