It's a sequel on the same platform. Sequels of the same franchise on the same platform often sell less. On SNES SM:World = 20 million. SM:World2 = 7 million. On GB SMLand = 18 million. SML2 = 11 million. eg. God of WarYou mentioned New Super Mario Bros (2009) selling 28m on Wii. Impressive. Now look at Super Mario Galaxy 2 (2010), a game with a 97 metacritic score, which sold 7.4m on Wii. How do you explain that?
Sales of 2 were lower than 1 the same platform. GoW3 on PS3 sold more than either.
Or Gran Turismo.
Nothing unusual about that. Now plot the data of Mario's sales...
Is there really a downwards trend there? Remove Wii U and NES as outliers and it's pretty obvious that Mario has had very variable sales per title, but they have always sold significantly, 4+ million. Very, very few IPs can ever have a hope of 15+ million units on one platform. All the big selling titles on MS and Sony consoles like GTA are cross-platform. Minecraft, on every platform under the sun, has apparently sold less than Mario Kart Wii fer cryin' out loud! The only game with one platform in that list of top selling titles that isn't on a Nintendo is Kinect Adventures, which dropped like a brick in its sequel.
How can you look at all this data and still conclude Mario hasn't got staying power? Even if you want to excuse it as there be no other games worth having on Nintendo consoles, that still means the game has staying power - just as a product of its environment. It's a game that gets best-selling numbers. Sony's IPs on PS4 (sequels and remakes) are around half Mario's numbers on a much larger platform. Is Infamous going to be pulling the same numbers in 10 years? No. Only an IP with significant staying power can remain relevant for that long.