Sure, there will always be people who want to build detailed models (and not just of trains). But chew on this:
a) I tried to sell simple, shake-the-box freight car kits to a father (30 something) and his son (12-ish) last year. No go. They preferred to buy two RTR cars by Atlas instead of four kits by Accurail. They did buy a simple building kit, though. But only because I had no ready-builts in stock. IMO, they are a portent.
b) Walthers is producing more and more ready-builts and RTR. they no longer make or offer craft kits. They used to be a major source of craft kits for the "traditional multi-talented modellers".
c) Labelle and others (Mainline, Silver Streak, etc) used to be hobby shop staples. They've either disappeared, or been reduced to direct-to-consumer selling. And the cruel fact is that it takes a heap of effort to convert one of these craft kits into a model that even at a distance looks as good as its plastic RTR counterpart.
d) The better quality craft kits available these days are limited run, very pricey, and clearly targeted at a very small (but very dedicated) niche market. By "better quality" I mean those that will result in models that rivals the RTR ones.
e) My fellow club members, all of them retirees in their 60s and up, are increasingly buying RTR rather than kits, because they don't want to spend the time to build. They also note that the price difference between kits and RTR is shrinking, non-existent, and even reversed. When a resin boxcar kit sans trucks and couplers costs more than an Atlas Trainman car, what do you think they will buy?
f) Recent newbies in the hobby hereabouts prefer ready builts and RTR, and buy kits only when they can't get anything else. They're quite happy to pay the price premium.
IMO, the main reason MR is losing readership is the same as the reason all craft and hobby magazines, all magazines in fact, are losing readership. It's several reasons, actually, if recent studies of people's reading habits are valid (and I see no reason to doubt them.)
a) The Internet: Notice how many people here want a website about some subject or other, a subject usually well covered in books and magazines easily available at the drugstore and/or the local library and/or the local hobby shop?
b) A couple of studies done by industrial psychologists indicate that most people reduce their leisure reading from about the mid-20s on, so much so that their reading skills in general suffer, and hence their work performance suffers too. One of the reasons for this reduction? The astonishing increase in the amount of reading that must be done as part of the job, which averages 2 hr/shift for all job classes. People just don;t want to read more when they get home.
c) The pressure of time. People expect to and try to do many more things than they did some decades ago. So they for ways of reducing the "prep time" for each activity, so as to concentrate on the essence. The essence of model railroading is running trains. Building models is part of the mix in model railroading, but for most people it's something you do because otherwise you can't have the trains you want, be it for budgetary reasons or for some otherwise unappeasable desire for some specific (often nostalgic) prototype.
Final comment: the differentiation between "traditional multi-talented modellers" and "the entry-level ready to run crowd" sounds more than a little elitist, and does the hobby no good. If you want the hobby to thrive, you can't afford to alienate the newbies with that sort of disdain.