I think you are thinking into this a little too much.

The "Prescribed Visibility" is the visibility on the approach chart. The FAA determines the minimum safe visibility they believe you need to have to NOT be in danger and they write it on the approach chart. The SM visibility and the RVR are telling you what FLIGHT VISIBILITY is required for that approach.

If the chart says a certain RVR and the RVR sensor is inoperative...

91.175(h) - Comparable values of RVR and ground visibility. (1) Except for Category II or Category III minimums, if RVR minimums for takeoff or landing are prescribed in an instrument approach procedure, but RVR is not reported for the runway of intended operation, the RVR minimum shall be converted to ground visibility in accordance with the table in paragraph (h)(2) of this section and shall be the visibility minimum for takeoff or landing on that runway.

RVR (feet) Visibility (SM)
1,600 1/4
2,400 1/2
3,200 5/8
4,000 3/4
4,500 7/8
5,000 1
6,000 11/4

In order to descend from an MDA / DA, the flight visibility you see out of the airplane window must be at or above the visibility or visual range (RVR) shown ("prescribed") on the approach chart for your category.

Believe it or not, it really is that easy!

Now, arguing "flight" visibility vs what the ground sensors are reporting is a whole other topic and is significantly different for GA / Part 135 / Part 121 airplanes.