It doesn't have to do anything with how likely what could beat it or what not.

I reported him for calling me delusioned for my opinion.

We get it, spemanig. You don't like Splatoon!

The audience they sold to doesn't discount their existence.

I agree 100%.
Said it before, I'll say it again.
Although the Wii U is a failure, it's not a total failure. Splatoon is the reason for that. It has successfully become a new powerhouse franchise under Nintendo's banner, and the Wii U is the machine that brought it to the table. Splatoon alone will ensure that the Wii U is remembered forever by not only fans of Nintendo, but gaming fans in general.

The Wii U won't be remembered because of Splatoon, it will be remembered because of the disaster that was the Gamepad.

In Japan probably, but on a global scale, I don't think that would be accurate. Great game has done wonders but it really hasn't defined or shaped anything other then being very successful, I think Mario Kart 8 or Super Smash Bros. for Wii U are representative for the Wii U's life cycle.

It's either this or Mario Kart 8 DLC Packs.

Did you actually read OP!? Gamepad is not a game.

How exactly!? Completely new, fresh and great idea in TPS online field, you dont have nothing similar like Splatoon. Splatoon has come out of nowhere and surprised us all and become a massive hit, its also completely new IP with own characters and world that become very popular with toys, music, mangas...while Mario Kart and Smash Bros are games that were great and successful games previous 3 or more generations.

But a system defining game is more then being fresh, new and successful, it has to embody the entire life of the console. Don't get me wrong, Splatoon comes close, but much like Perfect Dark, Super Smash Bros and a case can be made for the THQ/AKI wrestling games are classics on the N64 and some remember the system for it, the three games that are system defining for the N64 are Super Mario 64, Ocarina of Time and Goldeneye 007. Splatoon is close to Goldeneye 007 but not as successful outside of Japan enough to make strong enough dent to save the Wii U, but the reason I don't think it will be what the Wii U is remembered for is that the massive fanservice of Smash Bros and Mario Kart tell the tale of the Wii U. Great games that did almost everything right sold very well but even with their weight (and Smash being the primary introduction of Amiibo) couldn't right the ship that was the system.

To me that pretty much sums up the Wii U's life right there and Smash Bros with its DLC, Amiibos and overall heavy fanservice across the spectrum just seems to be what I think most will remember the Wii U for even if we get another game in that franchise that has similar roster size and diversity.