My view is that any character in this game is viable as a choice for a main... but more importantly, I actually view Smash 4 as a game where playing multiple characters is almost necessary to succeed at the highest levels.
We discuss the game engine itself at excruciating length, debating how fit for competitive play it is... but I feel like other important aspects of the overall competitive environment are totally lost in that discussion. To me, the competition starts before the characters even start fighting, before the game engine even comes into play. What makes Smash 4 a great competitive game to me is that, while there are certainly stronger and weaker characters, I feel like every character is capable of certain things that make them worth putting practice time into. Picking a character to counter an opponent's character or even their playstyle is more important than in any previous Smash, imo. Because not only does every character have different abilities, but I also think most characters can be played pretty effectively in a variety of different ways. There may be consensus on the "optimal" way to play a character, but that doesn't mean doing something else is totally unviable, and in fact can be beneficial either because it's unexpected or because the "less optimal" playstyle is actually very effective against your opponent's character and/or playstyle.
And then, of course, there's adapting mid-match to gain an upper hand. Because most characters have enough tools to be able to try a couple different things, you're always able to adapt on the fly to put yourself in a better position to win the match. For me personally, Greninja is my secondary behind Peach. Most people, I think, view Greninja as a primarily defensive character, making use of Water Shurikens and spaced fairs to force your opponent to approach, at which time Greninja can punish almost anything that isn't executed to perfection. However, I tend to play far more aggressively than that. (And not just with Greninja... I like to play almost every character from time to time, and I play offensively with every single one. Of course that doesn't have to mean rushdown, depending on the tools the character has available.) I still do well playing Greninja more aggressively, but the real key for me is flowing in and out of an aggressive approach and zoning with Shurikens. I've won so many matches by, as soon as I see my opponent adapting to my aggression, switching to a more defensive zoning game, and changing between styles whenever necessary.
One of the great things about this game to me is that this sort of thing is possible with every character. Even when I mess around with someone like Zelda or Wii Fit Trainer, I rarely think "wow, there's literally nothing I can do here." Of course there are some polarizing matchups to where that's basically true, but even within most of those matchups I feel that it's often possible to compete very well by playing intelligently and using absolutely every tool at your character's disposal.
So... that probably doesn't really answer the original question lol, but it does in the sense that I'm saying, yes, Greninja is a perfectly viable main, and so is every other character in the game. At the same time, no matter WHO you main, learning a few characters, and different playstyles with the same characters even, can really do wonders for your overall competitive viability. I personally have around 6 characters I'd count as my main and strong secondaries: Peach, Greninja, Mega Man, Bowser Jr. (actually Lemmy haha =P), Mewtwo, & Charizard. And I play most other characters relatively competently, though of course you can't really master everyone. But if I'm playing a tough opponent, I'm completely comfortable switching off of Peach to any of the other 5 if I feel something about the match warrants it, and being able to do that makes a HUGE difference to whether or not I'm capable of beating someone.