Wii Sports, Super Mario Bros/Duck Hunt, Super Mario World, and Tetris aren't killer apps. They're introductory games and they did their jobs perfectly--they got people addicted to the consoles. They showed them off.  They gave other developers ideas as to what the systems could do.  They were glorified tech demos to sell a console.  That was really their only job and in that regard, they were all extremely successful. Just because Wii Sports has seemingly high sales doesn't mean it's a killer app, it means it's a successful, bundled, introductory title.  The major difference between Wii Sports and SMB or SMW is that Wii Sports is barely more than "just a tech demo," while the Mario games were full-fledged titles.

A killer app is a game that both makes owning a certain system worthwhile and is often a major contributor to someone owning said system.  Super Smash Bros Brawl is a killer app.  Resident Evil Remake and RE4, along with Metroid Prime and Rogue Squadron were killer apps for the GC.  Final Fantasy VI, Donkey Kong Country, StarFox, Yoshi's Island, Contra III, and Super Metroid, in my view, are killer apps for the SNES.  The N64's killer apps were clearly Mario 64, Goldeneye007, Perfect Dark, Banjo-Kazooie, Zelda: OoT, and in my opinion, StarFox64.  Tech demo titles:  Blast Corps, Mario64, Pilotwings64, Wave Race 64. 

Killer apps are typically original, AAA-quality, well-reviewed, high-selling exclusives--I only list RE4 for GC because, for all intents and purposes, it was originally an exclusive title and you'd be hard-pressed not to find someone who believed that to be a GC killer app.  As far as I'm concerned, it's one of the 3 games you should own if you had a GameCube (the other two being Eternal Darkness and Metroid Prime 1).   Go back to the Genesis, Sonic the Hedgehog and the Phantasy Star games would be the killer apps.  Altered Beast was the tech demo title to sell the system.  Later in the SNES life, StarFox was both tech demo (as it was often playable in stores) and a killer app.

The first year of a system's life rarely delivers killer apps because these things take time to craft, so yes, it was foolhardy for people to dismiss the Wii early on for "lack of games" especially since of the current three consoles, it actually was the only one that delivered a killer app in the first year--Super Mario Galaxy, while it was well after a year before Bioshock, Halo3, or MGS4 hit stores for the other guys.  Anyway, the first year will generally have a lot of tech demo-style games intended to show off the system with killer apps coming after these newly discovered tricks are mastered.  For instance, in the first year of the SNES; SMW, Pilotwings and F-Zero were all intended to show off some of the feats possible therein.  Super Mario Bros was mostly intended to show how much more could be done on the NES than any previous console. 

Nintendo need not replace Wii Sports.  The only thing to do is add another game (or demo disk, preferably) to it in future releases to maintain momentum.

I think a big chunk of the confusion in this thread may have grown from you listing Wii Sports as a killer app which is exactly what that game is not. 

GTA V with super gameplay or KH3 or Wii Sports 2 with online like Bursche said!!

Diablo 3 Wii Edition

If/when Nintendo releases a Pokemon MMO, Sony and Microsoft might as well surrender.

Any true sequel to FF, DQ, or KH because of the retardly HUGE and dedicated fanbase.

@Resident_Hazard----> So whats the game equivalent of Wii Sports and Super Mario Bros. for the original PSX then? Whats PSX introductory game? Sorry never bought the PSX at launch so I never knew...

Their is no one,only game to rule them all.

 Hate to disappoint, but I can't say.  I was always more a Nintendo guy that also rooted for Sega.  I don't really remember much of the Playstation's early releases.  I think the original Tekken and Battle Arena Toshinden were on there, and Twisted Metal 1.  I remember playing Twisted Metal 2 with my friends all the time in high school, but by then, the system had somewhat more refined games.  The original WipeOut comes to mind as one of the early PSX "demonstration" titles.  The early Playstation games contained an offensive glut of PC ports and Doom-clones (before First Person Shooters had the refined modern control scheme first tested with Turok 1 on the N64), and some Arcade ports and things like that.  I can't list off-hand any actual Super Mario Bros/Duck Hunt introductory games although I do believe the system originally came with a demo disk.  Mine did at any rate, but I bought it when Tekken 3 came out some years later. 

The 32/64-bit era was really the first time consoles started launching without games packed in, which, in my view, was a huge mistake.  Prior to that, pretty much every system came with games packed in.  Go back even further to the pre-NES (and there about) days and consoles occasionally had games programmed straight into them though cartridge games were still seperate.  I remember actually being shocked that the N64 came with nothing and that I had to buy a game seperately when my NES and SNES, and my brother's Game Boy had all come with packed-in games.  The Saturn had an extremely rocky start and I don't think it launched with packed-in games, either.

As a side note, the 32/64-bit era was also when this concept of "selling the console below cost (at a loss) and make the money back on software revenue" came into the industry.  Sony introduced it and Microsoft followed suit.  Prior to that, all console makers, including Nintendo, Sega, and Atari, sold their machines for profit.  Nintendo is still the only company that does things this way.  Sega and Atari were both replaced by Sony and Microsoft, and they seem to like losing money with a goal of "eventually" making it back with games, although it's debatable if this strategy is really a winning one.  Microsoft didn't make a profit on the Xbox until over halfway through its short cycle, and Sony has lost literally billions on the PS3.  I've always believed that the game industry should be left to the game companies, which is why I was initially very against Sony and Microsoft entering the fray.  But Atari and Sega made shitty choices, shot themselves in their feet, and generally made poor business decisions and Nintendo pretty much did the same when they essentially handed Sony a winning formula to dominate the industry for two generations.  [/derailment]

Crash Bandicoot was supposed to be their Mario. :p Honestly, the closest thing to it would probably be Tekken.

 Yeah, but Crash Bandicoot wasn't a first-gen PSX title, it didn't launch until after Super Mario 64--and it had none of the strengths of SM64.  For instance, there was no free-roaming 3-D, you were constrained to a little path.  Crash was a more refined "later" game, not one to show off some of the capabilities of the PSX in it's first year.