Mercedes' SL sales really took off with the 113 series cars from 1964. They lost lots of the edge the previous ones had, but gained a much wider appeal. Mercedes' SL models since that point have not been about going head-head to with the 911, but about offering something different. Many of the richy-rich 911 owners I met through the PCA also had an SL as well as a couple sedans, often an E-Class and an M-series BMW. 911's were track cars and weekend machines, SL's often driven to work, M3/M5 for hauling people and an E430 Wagon for the family. My attorney had a 911 "toy" and an E36 M3 "daily driver" with an S500 "family hauler" and a Suburban for towing stuff and the nanny to use. When the new M3 (E46) came out with even more race-car character, he bought a new SL500 for his daily run-about, liking the comfort and new styling.
Mercedes has tried to offer manuals in North America, and without much success. Even in Europe, the larger models have the demand for slush-boxes.
I can't stand the brake-by-wire (though I haven't driven the SL's system, only E-Class) and hope they sort that out soon.