I hate to bring your argument to a close, but if you search up the early results and/or you find videos explaining why Inkling should and will be brought to Smash, then you'd understand or you'd be in full denial.
@SuperMii3D has a point, and it is one that I came here to make as well. It doesn't matter how well Splatoon does; it matters how much they promote it until the very end. Inklings already got an Amiibo. Do you think giving one a spot in Smash would be so hard? Nope. And even if you believe that the attacks would OP, like SuperMii pointed out as well, they have a team dedicated to balancing the fighters.
As far as saying an Inkling doesn't fit in Smash, I present you: Pac-Man. You can argue that Pac-Man is a 'pioneer' of video games, but why would he fit in Smash? And if we're talking about old video games, then where's the guy from Pitfall? And why isn't E.T. in the game? (Okay, E.T. was a terrible video game. But it was still an old game!) Now that Pac-Man is in Smash, it's obvious how he fits because of his moveset. But beforehand, would you have thought he would make a good fit for Smash? It's all about attempting, and if enough people vote Inkling in, then it'll happen. And I know that plenty of people have already voted it in.
Ice Climbers were pulled because they weren't used enough in online matches in Brawl. Roy was pulled to make room for Ike, because they didn't want to take the chance of presenting more Fire Emblem characters in the series if it wasn't as popular as they wanted it to be in North America. Obviously, Awakening attracted a new audience (3DS players) and thus we were given 2 more Fire Emblem characters. Snake and Lucas were pulled for the same reasons as the Ice Climbers; and I have no earthly idea why they're bringing Lucas back. Maybe they can rework him to make him less of a Ness clone, like they tried to differentiate Mewtwo from Lucario, since Lucario replaced Mewtwo in Brawl.
You're correct, maybe there's not much info to 'verify' that Splatoon will be a best-seller, but there is plenty of evidence to state that Splatoon is a very much hyped up and talked about game. Nintendo is about making investments that they believe will work best, and had they thought Splatoon wouldn't be popular, they wouldn't have given Inkling an Amiibo. So, no, there are no real statistics to clarify how well Splatoon will do, but there is a great deal of hype for this game and I imagine that if people continue to vote for Inkling, it will bring a huge variety to the Smash roster.