Here we go again with the "Flyer is bad for S Gauge" chant. S gauge wouldn't exist today if not for Flyer and ACG's adoption of it! I don't here the 0 Scalers chanting that "Lionel is bad for 0 Gauge" and knocking that three rail track. Face it, the "casual modeler" (i,e, those that just want to run trains) is the largest marketplace for any trains, and their purchases make possible the neat scale stuff that follows.
As to Steamers "ensconce the Flyer name deep in the past" I don't see that effecting manufacturers in the other scales, who sell LOTS of different kinds of Steamers, along with modern and not-so-modern diesels. Most model railroading is nostalgia, and stuff from the past fits in. YES, longer, modern freight cars would be good to have for many folks, and contrary to the L engineers' thinking, they will run on standard track, although you will have to have truck mounted couplers and be happy with extra space between cars, like the AF passenger cars. Around these parts you mostly see container car trains. I would think a container car would be a fairly simple car to make, and the containers can be sold separately as loads for other cars and track side "scenery."
I do wonder how much more it would cost to have a detailed floor instead of the stamped metal floor on boxcars--but, one seldom sees the bottom of a car, unless they pick it up to look at it! One reason I've not fully understood the exacting attention to detail some modelers do to the "under floor details" on their work. Incredible work, but to what end? ("My car's better'n yours; see, if you apply air to this brake hose, the linkage actually works!" "Oh, do your locomotives have air brake supply?" "Uh, no, but look at this detail!"--OK that's kinda extreme, and I would be amazed at it, but it's just an example.).
Somewhere this begins to smack of the "I OPERATE Miniature Railroads; you just PLAY with trains" conundrum. Let me make this clear, before you haul me off to the asylum; I play with trains, sometimes even the full-sized ones! ("My name is David and I play with trains." "Hello David!" -- TTA meeting (Toy Trains Anonymous); Where's Mr. Doakes when you need him?