I think the obvious answer is that the 645 is a fine camera and design as well as format HOWEVER, the 67 (6x7) covers the 645 (6x4.5) format while providing more area of coverage for those who want it. Those who don't can simply crop to 6x4.5... So, if you're going to produce a
MF digital camera it makes no sense to develop the smaller of your two formats as all things being equal, you'll pay slightly more for the larger sensor but gain far more versatility.. My money is on a 67D to replace the 67II...