I have thought of this as well, and always come back to the all steel 6 shot .38/.357 SP101 camp. A (.357 and 9mm variant) D frame sized LCR would fall in between the J frame sized LCR and SP101 in weight. This would cause a lot of internal competition and would not be good from a marketing standpoint. A 6 shot SP101, with the aforementioned changes, would be about identical in weight to the second frame size (allowed longer than 125 grain bullets in .357) and still take advantage of almost all SP101 accessories. It would also renew interest in an older product line and perfectly fill that gap bewteen the LCR and GP100 from a marketing perspective.
Recent production alloy frame Smiths have not had the best reputation. The stories on the S&W forum are both entertaining and horrifying to read. The beauty of an all steel D frame sized gun is durability, comfortable practice, and a greater ease of carry compared to medium frame guns. Also, the Model 12 might be light, but it is still dimensionally a medium frame revolver. The lock is not going anywhere, anytime soon. Some folks say this is due to lawsuit fears, but Ruger has removed locks from some models and did not get sued into oblivion. I have always thought that the main reason S&W will not do this, is due to the frame shape changes. If they removed the locks, they would be obligated to go back to the original frame shapes and they don't seem keen on spending much coin on their revolver line.