Could you elaborate more on why propagation is not the right term? Its definition reads:

  The process of spreading to a larger area or greater number; dissemination.

Which is (to my understanding) exactly what happens with DNS information while clients fetch the updated zone.

72 hours is indeed a comfortable upper bound ("up to"), that we would give to customers so as to make absolutely sure every cache down the chain has refreshed its records, and is not the cause of the issue at hand when name servers were changed. 24 hours is common delay.