Lots of great comments here thus far.
The only think I'd like to add... We can't underestimate the fact that postwar trains were desirable because they had a very finite, well-defined period of production. AND folks who may have missed out on them as children now had a chance to grab some very nice pieces in great condition -- albeit at premium prices. The FM Train Master locomotives are a great example. They often sold for under $65/each in the 1950's and 1960's, and by the 1980's the rarer models were commanding 10X-15X that number. But that's largely because they weren't being made anymore. True... Lionel started re-issuing some conventional models in the 1980's (at roughly $150), but the more rare road-names and variations from postwar still held their values.
By the 1990's, the technology treadmill kicked into high gear. And we began seeing command-control locomotives with better sounds, and more details. And we've yet to step off the technology treadmill. Although advances are somewhat more incremental now, it's still there -- and has its effect each and every time a re-issued model obsoletes an earlier offering.
Add to all of this the fact that the demographics of our hobby are changing day by day, month by month -- be it aging, health issues, downsizing, whatever... all the stuff mentioned in earlier posts -- and I think you can see everything coming into focus: it's a "perfect storm" of sorts that has resulted in much, MUCH lower demand for postwar Lionel trains.
Think of it this way... if you visit the Allentown ATMA show twice a year, you'll see guys hauling tables upon TABLES of old post-war trains in and out of the halls every show. (Presumably the same phenomenon happens at York too, but I spend all of my time in the dealer halls.) People are finally realizing the amount of space their trains are occupying -- whether on layouts, display shelves, or storage closets. And there's just no more room for anything but the most desirable pieces that fancy enthusiasts at any given point in time. A like it or not, postwar trains just don't rank high on most peoples "buy" lists nowadays.
I'm not too sure it's strictly a toy vs. scale thing either, because Standard Gauge Tinplate has a toy-like charm all to it's own. And I sense a call pulling me in that direction at times. Not enough to operate them at the expense of real-estate on my newest O-Gauge train layout, but tinplate has a special charm even if it's just for display purposes. Stay tuned in a few months, and I'll show you what the custom-builders ended up doing with Standard Gauge displays built into my O-Gauge toy-themed layout. The colors will jump out at visitors like a feast for the senses. Can't wait to share it with everyone.
So while postwar demand may be down, I wouldn't write off the toy-like charm of our modern-day trains just yet to pure scale interests! Toy vs. scale is certainly part of the reason the postwar models have seen better heydays. But a well-built, toy-themed layout (with a mix of old and new technologies) can still deliver quite the WOW-factor in its own right.
David