One important point:
I think that Siemens is ditching the MPI/K-bus part of the backplane.
A strong indicator that this may happen is that the new IPC in S7-300 format (S7-mEC) only supports the regular SM i/o modules. Not the FM or CP modules.
There is also no MPI or DP port on the CPU, only Ethernet ports.
I find it very good that MPI is dropped in favor of Ethernet.