80 stages is like an expansion pack from the old days of PC gaming. This isn't just like some minor thing they clearly deliberately left out of the game to ding everybody, this is substancial.
If Nintendo is going to release cookie-cutter sequels this is the better avenue to go down. Release a decent sized expansion pack as DLC for a lower price than a regular game. I imagine this would be a little less involved for their teams than a full on new game, it's cheaper for the consumer and I figure that since it's DLC Nintendo will still feel pressure to fill the release schedule with new standalone retail games.
Though, uh, yeah, NSMB overload here. I remember in the 90's console wars defending Nintendo on the basis that each Mario game was quite distinct from each other and offered a notable improvement, while Sonic's sequels were far less creative and released far more frequently. Well THAT sure doesn't fly anymore. But then I can say that Mario games are good and Sonic games are not so I guess I still win the Nintendo vs. Sega debate.