Don't many of these examples of future bafflement require some sort of humongous historical discontinuity?
Yes--but that's because that's what happens. There are a lot of artifacts that took a long time to understand, and many we still don't. Then there is the social aspect of much of it--without a working knowledge of the society something was found in understanding what it is is often impossible. Let's say you find a bit of braided copper wire. Most of us would think it's trash, or leftovers from some electrical work. But it's not--it's a circlet worn by someone in the SCA. Without knowing about the existence of Medieval re-enactment groups, you completely screw up your interpretation. Similarly, knowledge about subcultures, counter-cultures, and even aspects of that culture that don't have any correleation to today's culture is necessary to put artifacts in their proper place.
This is why I deal with animals. You find a dire wolf bone and you know what it is!