I have a question you might be able to answer.

AC 90-108 Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures discusses when you can and can't use RNAV systems to substitute for an inoperative NAVAID.

For example, you can't substitute an RNAV system for a out-of-service NAVAID when the navigation or procedure is NA (not authorized), when substituting for a final approach course, or substituting for a LOC or BCLOC final approach course.

With that being said, with the invention of RNAV RNP procedures, why would you still need to have raw data on a final approach course? The tolerances for an RNP approaches are tighter and safer than raw data VOR or NDB. Most operators today fly VOR and NDB approaches in LNAV/VNAV or similar fashion. If combined with RNP procedures, who cares if the NAVAID was working or not? Maybe not the right question. The FAA apparently cares. How about, why do they care? What am I missing?