The original poster asked an honest question. My best honest answer is I don't think things are necessarily headed the way he thinks. In particular, there is, imho, little prospect of Phaseone, or anyone, having exclusive access to whatever MF chips is cooking up.
Sony's sensor division is, apparently, mandated to be an independent profit centre, to the extent they gave Nikon their prime 35mm chip ahead of their own manufacture. This decision can only have been on volume and the profits therefrom.
While Phase has a nice niche convincing people to pay a $28,000+ profit margin on a back, I doubt they can muster the volumes to interest Sony.
Moreover, it is very unclear that any appreciable number of shooters want back of much more than 50MP. Very few people need or want these chips. Not to say some don't, but the numbers are a small, small subset
On the other hand, it seems likely that a very high MP 35mm chip will be released next year. That will cause much sturm und drang, no doubt.
The upshot is that the older digital backs are becoming good value. If you want high MPs, get one of those.
Happy shooting.
- N.