"Maybe the ATC controller was following the book, but my question later was - if the RVR was reducing below legal minima one dark and dirty night, would they not tell me until 10 minutes after I had crashed on the field ?"
You won't crash on the field because the vis has dropped. However, you might crash on the field if you don't react correctly to the fact that the vis has dropped. All you have to do is carry out the correct missed aproach if you don't acquire the correct visual refrences at MDA/H. It is not rocket science, it's aviation...it's your job.