Here's the deal, as I see it: A lot of old bikes do not currently hold any value, but the value is certain to go up, for 2 reasons:
1- they ain't making old bikes anymore
2-they don't make 'em like they used to;most of today's bikes (the old bikes of tomorrow) are simply not as durable as the old ones. In fact, i suspect that the mid-market steel bikes of today will retain more value than the carbon superbikes. I suspect Ti will hold some value, too. Perhaps the most, as they are expensive when new, and not as common on the market.
I also suspect that mass-produced US-made bikes will hold value. Like S&M bikes. Custom US-made bikes have some advantages in terms of exclusivity, but the fact that they'll be custom, plus the fact that fewer ppl will be searching for a particular maker, will diminish this somewhat. These are all just theories, of course.
The problem, as I see it, is that a little knowledge can be a terrible thing. So, the guy with 1/4 of a clue sees an ol' Schwinn on American Pickers that's valued at a bazillion dollars. He figures, "hey, I have an old step-thru Varisty in the shed; it's prolly worth a mint,too!" so he lists it for ten times what it's worth ($300) and some guy with an 1/8th of a clue buys it for $200, thinks he got a steal, and then goes on CL seeking a guy with 1/16th of a clue to buy it for $400, and so on. Which, of course, in the end just serves to drive up bike prices in general. Which is why I hate the act of flipping, even though it seems like a popular pastime among RRB ppl.
Just remember, next time you're perusing CL for bikes you don't want to own but just want to flip, and you want to start whining about how nothing's cheap: Flippers have caused this sorry state of affairs.