I remember back before Splatoon came out we would have endless debates as to whether or not releasing content over time via free updates was a good move instead of just delaying the game and releasing everything at once. I've wanted to play this game asap ever since I saw it, so I argued for it's release method, and now that it's all said and done I feel that really was the right move! Really there was nothing but positives behind it. Those really interested like myself got to play the game sooner rather than later, it got to release during a time without a notable release, the updates encouraged people to keep coming back to the game, and those who felt there was too little content at launch could just wait until enough arrives. Everybody wins!
The game's great sales don't necessarily have anything to do with this, but I'd wager the updates helped it's legs, since if people are still playing the game, they're also still talking about it and getting their friends interested. No real proof of course, but with how well the game has sold I think it's safe to say it had an impact, atleast a little! Plus again, it got to release on it's own while a delay would probably have put it the middle of the busy holiday period where it wouldn't be able to shine on it's own.
Alright fine I lied, there was one downside to this method, and that is it's review scores. It still managed a perfectly acceptable 81 on metacritic, but there's no denying the score would be higher if the game was reviewed as it is now, or, you know, if reviewers didn't write reviews they knowingly knew would be made irrelevant days later, but I digress. I have to give Nintendo major props here, they basically said "screw the scores, we're doing what's best for consumers!"
But that's all my obviously biased opinion of course, though I wonder if anyone still disagrees and would of rather had Nintendo delay it.