It's being replaced because it was a solution for a problem than nobody had (a tiny workstation without any expandability). "R&D" is a sunk cost - you don't replace something after six years because it cost too much to develop seven years ago.
You replace it because it is a lemon that isn't selling, and your most prized customers are buying Z-series by the truckload.
They would need to redo all of the designs for it. Its why it stagnated after 1 iteration. New videocards would need to be designed to be part of the design. New chipsets for cpus motherboards would need a custom design to fit in the case. What if the thermal load and power supply needs changed? New design. All of that is extra r&d - every 18 months - forever. They are ****ing morons for doing this in the first place. And they forever lost my respect as an innovative company for doing it. Steve is dead and so is apple.
I use mac everyday to help me with my work. But thats all this company is to me. Im happy to buy the new one but this trashcan will never be updated again and whoever thought it up shouldn't be working.