The U.S. had C130 ABCCC for many years, also SENIOR SCOUT is still around. Both modular the SS is normally taken up from a normal AT ac. The ABCCC, when it was in service, stayed on one platform as it was too expensive to swap over all the comms, however, now that would not be such a problem with small satcom antenna which fit into escape hatches etc. Other nations also have C130 special fits which role in roll out.
Seems to me to be a good idea which allows for flexibility. Buying the P8 does not have this option.