It really all boils down to $$s. What is selling and what is not. As pro's move their purchases to the R and fewer to the 1D series, the cost margins go down, they will move to abandon the line.
I'm still using a 1DIII and 1DsIII and love them. I love the feel of the 1 series body. Some day I'll probably buy a newer 1 series used. Actually both of mine were used. I actually use my 1DIII most. Im still amazed how good 10mp can look even compared to the 21mp.