I have thought about this often and have come to the tenuous conclusion that eventually, an earthbound spirit will get absorbed into the land if there is not someone around to keep their memories alive. We don't see cave man spirits, and frankly I can't think of any stories of seeing ghosts going back much more than five or six-hundred years. Obviously this would vary per culture since there are ancestor-worshiping cultures. Just my thoughts.

The ancient humans I have always thought to be as intelligent as we are and in some cases more. an Example take most of us and sit us naked in the woods and tell us to survive not many of us would on the other hand, they could survive in our society.As far as their feelings I think they had the same as ours today as emotions are concerned .
I feel that there is much more to spirits then just residue energy I do believe this phenomena exists but not the total answer. The possibilities are endless.So the question remains the same Do earthbound spirits have a time limit? and if so what is that time line.

Others think the same way as you, sounds like a second death. What I would like to think is that they go on to a higher plane of existence. but in reality you are probably correct. Thanks