Сабж. Во всём, оказывается, виноват Emacs.
Emacs has no support for ImageMagick 7. Other distributions have attempted to patch Emacs to support it and have ended up reverting to ImageMagick 6 due to instability.
As long as ImageMagick 6 is still getting upstream support I don't see an issue with sticking with it. We know it works.