I don't see the problem. The camera is an open system so there will be adapters for current non-P1 backs. I see this decision normal because if you make the camera with Mamiya mount, anyone making a DB compatible will be making it compatible with the new P1 camera and Mamiya AFD automatically, and this could be to just give too much compatibility.
As Michael said, this camera is a P1 design so is normal that P1 wants their own mount. And the reason is not economical , because P1 owners (who purchased within 12 months) will get free back swap, and the rest of owners for a nominal fee. And their are closing none! Eventually, Mamiya will make a new back with the new mount and offer swaps like P1.