There are tons of patents regarding implementation details and instruction set extensions. I don't think you could sell Athlon 64 or Pentium D clones to anyone.
Not sure about the original Athlon 64, but Pentium D's can apparently run, e.g. Win10 for x86-64 and its associated software (barring ISA extensions, of course) - surely that's good enough for some users. Especially if you could reimplement the "clone" on a more recent physical node than an actual Pentium D.