RNP isn't just position input
IO540 - the concept of RNP is relatively recent, and it is worth remembering that it stands for Required Navigation Performance. The lower numbers of RNP in many legacy aircraft are limited not by the sensor performance (as you say, many modern FMS systems know where they are to better than a tenth of a mile) but by the performance of the rest of the system, including flight directors, autopilots, and so on. For example, older autopilots with limited steering authority from the navigation kit often can't meet the requirements for accurate tracking at RNP 0.3.