The big issue is the price of the sensor. I got a price from Kodak on a 22MP sensor a while ago and even in quantity the price was around $5K, iirc, so you can easily imagine that a ZD costs $6K+ to build.
I think you could be right. One story I remember was that Mamiya committed to purchasing a rather large number of sensors in order to get a good price: it might be then that with the 22MP sensors already payed for, Mamiya is now selling at a profit compared to the alternative throwing away the sensors, but at too low a price to be feasible when new sensors must be purchased for a newer model. Using your numbers, if Mamiya has already payed the $5000 sensor cost, they are $2000 better off making a product that costs an additional $6,000 to make beyond sensor cost and selling it for $8,000, but this would overall simply be reducing the loss from $5,000 per sensor if written off to $3,000 per sensor.