Movies are a reflection of the times, and in turn a snapshot of our social history. When a poster is both appealing visually and representative of a time gone by, then it will be desired. The connection is the shared human condition.
What do book collectors want? They want first editions. Comic collectors, currency, toys, furniture, militaria, folk art etc. Same answer I suspect.
What will pass is the '80s material. It will flow straight into nineties posters, and then linger in obscurity for another 70 years until they reach antique status. I'll put my house on that!
My cup is not half empty. It runneth over, ye doomsayers!