Consider constructions that have survived thousands of years such as the Tarr Steps or Stonehenge, or more recent constructions such as roman roads, Dunfermline Abbey or St Bartholemew's Hospital.
These sorts of constructions have been repaired and re-set hundreds of times over their existence, and have sometimes gone through periods of destruction during war and natural disasters, disrepair then subsequent periods of restoration and reuse. At a certain point, very little or nothing of the original construction really remains, but you can nevertheless draw a line through hundreds or thousands of years of history.
Software may be more like this: continually rebuilt and maintained, but still physically or philosophically related back to some original construction. Nobody uses Multics any more, but almost everything in use today is derived from it in some way.
These sorts of constructions have been repaired and re-set hundreds of times over their existence, and have sometimes gone through periods of destruction during war and natural disasters, disrepair then subsequent periods of restoration and reuse. At a certain point, very little or nothing of the original construction really remains, but you can nevertheless draw a line through hundreds or thousands of years of history.
Software may be more like this: continually rebuilt and maintained, but still physically or philosophically related back to some original construction. Nobody uses Multics any more, but almost everything in use today is derived from it in some way.