No.. They knew they had a winner long time before..
Not only was splatton the most praiset game at Nintendo 2014 E3.. Nintendo UK also month before said it would be a true system seller..
BUT.. Spla2oon is a 2018 thing.. 2017 would be to early.. and Nintendo also need time to make it good..
No... lol they thought that it had potential, but they didn't knew if it would have been appealing.
It wasn't certainly the most praised game at E3 2014, most of the people was unshure about the game and it was outshadowed by Zelda, Mario Maker, Xenoblade and Bayo 2.
end of 2017 is possible is the only thing I'm saying, the game without counting all the concept work made before starting development took about 1 year and a half so it's possible for Nintendo to complete the sequel in 2 years+ a couple months
I'm convinced that Nintendo was sure that Splatoon would be a hit and it was the journalists and the general public that didn't pay much attention to it at first. Nintendo themselves kept on hyping the game with the giant booth that took up a good chunk of Nintendo's section at E3, the continuous segments they had during the Nintendo treehouse gameplay marathon, the Nintendo reps hyping the game up in interviews as the 'Mario Kart of shooters ' and the fact that they ended their digital event with the game. They knew they had something big on their hands when E3 2014 came around.
As for Splatoon 2, I personally think Nintendo needs to hold off on it for 2018. If they churn out a library in the NXs first year that includes significant games like Animal Crossing, a 3D Mario game, Zelda, Pikmin 4, Retro's game and Luigi's Mansion 3 then Nintendo can afford to wait on Splatoon 2 for a year. I could see Nintendo anouncing Splatoon 2 at E3 in 2017 and releasing it in summer, 2018.