Tom, look up Baro VNAV and you will find the biggest limitation to current RNP operations.
Until a better altitude measuring device is used RNP operations will not approach the accuracy and reliability of an ILS. WAAS and LAAS can increase the accuracy but it is the vertical that limits it.
Also human failure to report the correct QNH will affect the Baro VNAV solution. Human failure in setting, reading and transmittim QNH is about 10 -3 reliability verse FMC reliability at 10 -7.
Make no mistake I am a big fan of RNP AR operations and fly them almost every day of my working life in some very interesting places, but it still does not replace an ILS yet as a better approach. It is close but for most operations it is not quite there.