Even though I believe it is very good value for what it is, especially when Hasselblad and Phase One use the same sensor but charge many times more than Pentax does, I don't see the 645Z's running out of the stores.
The interesting part here would be: why does one want to go medium frame?
For raw sensor size/resolution the D8x0, A7r are, compared to the new CMOS-sensor, almost there. To make the step worthwhile one of the full 645 sized sensors would be helpful.
LS-lenses are an actual added value, at least with HBlad and POne; that can make sense from a bookkeeping point of view.
Some legacy stuff? Would be a reason, but tough to capitalize on for a new manufacturer.
And then there is the factor "prestige", either to impress someone or to caress ones own ego.