I agree to some degree, IMO his last true voice was the Australian tour, although I see what you mean with the 06/14 show, it really is one of the better shows for Plant on this tour. To see what I mean search for 'Rock and Roll live Australia'' on youtube. When I think of Robert Plant, I think of that clip.
That does make more sense regarding the reason for his vocal problems, I remember listening to a bootleg for the BBC concert in April and hearing him explain why his voice was so hoarse, wasn't it during the acoustic set ?
This was exactly the answer I was looking for, you can't lose your voice like Plant did by smoking cigarettes and doing cocaine, it's impossible.
And sorry for the poor writing structure, English isn't my native language and my sentences can sometimes be hard to read.