I'm guessing the take-rate for the DCT has gotten higher each successive year, as it's proven to be a good option, faster, etc. I'm sure sales of the early years were affected by bad memories (and reviews) of the SMG.
Think it will be a no-cost option on the new M3/4? The manual is a no-cost option on the M5. Pretty much a given that will be the final time it's offered in that car...