That's a good point, that S&W kept making the K-frame .357's for almost 20 years after the L-frame came out, and also the K-frame .38's which are still produced. S&W still makes the Model 10 in small amounts, people have said that Smith would drop the 10 for years but it still hangs on in the "Classic" series. The 64 and 67 are still widely available, despite the fact that "gun experts" say a 4" .38 revolver is totally obsolete. S&W doesn't seem to forget the guns that made them, and K-frames were S&W's bread and butter since 1899.
Ruger has a habit of trying to "clean up" it's catalog, by deleting certain models. There has been recent talk of dropping the standard Redhawk, because the Super Redhawk is cheaper to make since it's basically a GP100 on steroids, and fills the large frame big bore Magnum hole in the lineup. It seems standard Redhawk production is in "batches" now, and I haven't seen many of those super neat 4" Redhawks for sale.
I guess we should be glad Ruger made over a million Sixes, and they still circulate on the used market. If they had stopped making them after only a 100,000 or so they would be $1,000+ guns.
Ruger also dropped the .454 Alaskan right after "Faster" came out, with The Rock using one for the entire movie. Ruger would have sold quite a few just because of the movie, since it's been proven that movies sell products. Guys would have bought one just because The Rock used it in a movie, but now you have to find a dealer with one in stock or a used one. There's also that picture of the guy who killed a 900 lb. Brown bear with a .454 Alaskan when it charged him while he was bowhunting.
Ruger has dropped some great guns, and then people scream for them to come back. Ruger has said that the SP101 fills the role for a smaller .357,plus the small amounts of 3" GP's that come out, and thus the Six series was not needed. Maybe someday Ruger will start a "Classic" series like S&W did and make some Six series guns........we can only hope.