Imho 72M will be the Lifetime achivement for the 3DS, I now and then hear people talking about buying a new regular DS so I think the 3DS will have similar legs, Pokémon Sun/Moon will also have a key role in selling those few milions units because people have to buy the 3DS to play the latest entry until the franchise makes the Switch (which is not guaranteed to happen with the next game, we just heard rumors, Game Freak's trackrecord is completely different).
Also to the people saying Nintendo won't produce any other 3DS unit: you're out of your mind. The 5 year cycle of the 3DS has long finished and despite that we are still getting games, Nintendo wants a secondary source of money when they move to a new console, especially when the previous device was a failure.
