I've read a lot of interesting theories on bikes and what makes them collectible. I think there are a lot of factors, and of course, super-rare, high-quality/limited run bikes will likely build value as they age, assuming they're kept nice.
That being said, for cheap- to mid-level bikes, those that sell in very high volume to kids are most likely to build value. Why? B/c few of them will be kept in tip-top shape, but sooooo many potential buyers will have fond memories of their childhood bikes and look to buy them. Those little kids on OCCs in the 2000s will (hopefully) be wealthy in the 2030s, and willing to pay a fairly nice price for a slice of nostalgia. Once that ball gets rolling, all of y'all flippers will start buying them at low-ish prices in hopes of selling them for high-ish prices. This will promote the snowball effect, and then all bets are off.
Meanwhile, awesome bikes like the US-made S&M LTF will probably be collectible, but possibly not as much as OCC. Unjust, but understandable.
Think I'm wrong? Consider the fact that Schwinn sold, far and away, the most bikes out of any US manufacturer from ww2 up to the 1970s. Arguably, they are as common as it gets. Yet, they build more value than most of their contemporaries, with a few actual rare, unique, and awesome bikes (like Elgin Bluebirds) being notable exceptions.
Right now, as the last vestiges of US manufacturing are slipping away, the China vs US-made thing seems hugely important. It's very important to me, as well, but it remains to be seen whether or not this will have an effect on future collectability, or what effect it will have if it does.
