I think there will be increasing, not decreasing, value for mid-level, steel bicycles made in the US and Britain from the 1940s-70s. The fading away of those countries as old-line manufacturing powers, and perhaps eventual sinking to second-rate power status, will eventually yield a nostalgia for the products that came out of the height of those former industries. Don't necessarily hold your breath about a 1970 Schwinn Varsity, but a Super Sport, Sports Tourer, or something with a little hand-made cachet will do well.