My POV on a new Phase One/Mamiya camera is quite simple... P1 where smart enough to make their backs accessible from many users of different cameras and these backs can are interchangeable with older camera bodies of the same maker... Clearly there are many users that buy their backs on other than M645 mount and there are others that upgrade their backs while keeping their older M645 camera.
So, it all depends on how many sales does the camera body do... If it is thousands, a new camera body may be considered, if it is a few hundreds, It maybe wise not to bother... After all, a new body should be up to H5X level of quality to be worthwhile and that should require a good investment to be achieved... (otherwise I see no reason why one shouldn't consider buying an H body with a P1 back - many do).
That said, I believe that P1 would have raised their share in the market further if they would have supported Rollei 6XXX & HY6 cameras... I also believe that P1 would be even stronger in the market if they had multishot backs on offer and if their backs where adapted to different cameras via interchangeable plates... IMO, if Hasselblad wouldn't have closed the H system in the past and if they would have kept their CF interchangeable backs in production... P1 would have been a market follower of them by now, I also believe that if Pentax ever decides to offer a higher end version of their 645 camera with interchangeable back and even offer the back in more mounts than their own, P1 will lose the vast majority of their current customers to them....