I think we are seeing a small but real upswing in the value of used bikes across the board, so in that sense I agree that 70's bikes are becoming more collectible (at least in the sense that that they fetch more money than they did a few years ago).

But collectibility nearly always is a function of rarity. If something was mass-produced (say, like a good quality Nishiki or the such), then it likely will never see a real spike in value in our lifetimes. Market spikes are caused by market runs. If buyers get the sense that they are about the miss the boat, then prices suddenly inflate. But, I suspect that there will always be another dusty old Nishiki hiding in the next garage.

jim