It is an old question and only the only real answer is will photographers continue to purchase the IQ250 when the P645z does ship. There are some things to take into consideration however.
1. The Phase One backs, all the way back to the P45+/P25+ family of mid 2007 early 2008 were very well made. I have owned several different backs and have always felt that they were rock solid. Sure when one needed service, it had to most times go back to Denmark, but Pentax will be going back to Japan, as they don't currently have a service center in the U.S. (not sure on Europe). The Value Add warranty from Phase is a great asset to the owner and it moves with the back if you sell it. 4 years plus the base warranty and during all those years, a loaner back delivered to your business most times within 24 hours. Sure you can purchase 2 645z's, but to me that is not a smart move. It behooves you to purchase a Phase One back from a authorized dealer as that is the designed front end. It's very hard to reach out to Phase One directly, that is what the dealer is for besides many other things.
2. The DF+ body is long in the tooth I agree, but for landscape work it did OK for me till I moved on to the tech body. The body of the 645z should be much more advanced in features for sure.
3. The 645z is a fixed body, thus no ability to move to a tech solution. The ability to have a removable back, which allows me to work with a tech camera/lenses is an important feature. Pentax has a bit of a hill to climb on lenses. Currently their line up is in a bit of disarray. They seem to bringing back the old FA series from 2003-2008 as these are now listed again on the BH website, but the only current modern wide is the 25mm which is as much as the Mamiya/Phase 28mm.
4. If you are looking for a 1/1600 flash sync, then I don't think Pentax has any LS style modern lenses. There are some very old 67 LS lenses, but not sure if they are going to adapt to the 645z and allow the 1/1600 flash sync.
6. Tethering solution, Phase has this figured out, period. Still not sure if the 645z will have a USB3 tethering solution, if so who's software. So far all they have mentioned is a wifi solution, but that is not the same thing, as there is no way you will be able to transfer the full files via wifi. The 645z has USB3 so hopefully a software solution will available to tether. Yes it took Phase a long time to get to USB3, but they did! and it works! excellently.
I realize there is huge gap between the two, but only purchases over time will determine if the gap will close any due to price reductions by Phase One.
Paul