Why is everything still tied to the old 50 MP sensor? It's not even that it's 50 MP, which is plenty for most things - it's that it's an old, pre-BSI sensor. A fully modern 50-75 MP MF sensor could avoid the premium pricing associated with the 100 MP sensor, which may also have cooling questions (the big body of the GFX100 is partially to hold the IBIS unit, but I thought I had also read that power and cooling were part of it). Of course, Sony doesn't make a modern 33x44mm sensor other than the 100 MP, and there may not be the market demand for one.
There are presently four problems for any camera introduced using the 50 MP sensor - Z7, A7rIII,S1r, D850. Any of the four is capable of essentially the same image quality, with many more body features, and potentially a much more compact system. Hasselblad does have leaf-shutter lenses, an advantage in some applications. When (not if) a 65ish MP FF camera shows up using a slice of the 100 MP sensor (which is itself a slice of the 150 MP Phase One sensor, and the 26 MP sensor in the X-T3 is a small slice from one of these), it will almost certainly have better image quality than the old 50 MP sensor. From Sony's viewpoint, they'd rather make four physical sizes of the same sensor than an old sensor on an old process that competes in image quality with the very best of sensors one size smaller.
I almost wonder if the 50 MP sensor is already out of production - but Sony made a lot of them for some reason (maybe a big, later cancelled order for something we don't think of as a camera?), and they're sitting on a shelf somewhere, selling cheaply.
