But this isn't really how it works. The chips were getting faster, so you just do it along with the planned uplift changes. Most people didn't buy Athlon64s because they were 64-bit (except for us computer geeks) they bought them because they were the next evolution of a better processor than the competition at the time.
That was fine and dandy in the early-2000's when Moore's Law was still in full swing. Chip makers aren't practically guaranteed a compelling speed and efficiency boost just because a year or two have passed since the last gen, where it was a no-brainer to toss in 64 bitness since process improvements would gloss over any deficiencies or regressions.