Forgotten the numbers exactly, but isn't the required vis/RVR supposed to represent pilot eye height over the rwy C.L. i.e. 10m-ish. The RVR transmittometers were supposed to automate this measurements, but we all know their limitations.
Now the average ATC tower is 30m high. How can this relate to pilot eye height over rwy? If the pilot has the required visual reference above DA surely that it is appropriate to be able to make a decision to land. I agree about low level fog, but the same is true; if at DA and you have the visual reference you can decide to land. A mate at GRO did this, and at 50' it all went very dark and a G/A ensued. It still did not make the decision at DA wrong.