If Nintendo really didn't care about Splatoon, they would have dropped support as soon as all of the disc-locked content was shoved out. They wouldn't have bothered making anything else. Or, they would have released what they had done and then called it quits. Seeing as now we have new maps (I believe 3 or 4 that weren't already on-disc), a ton of new weapons (All Sloshers and Splattlings, as I believe their code was incomplete on release), and a lot of new gear, Nintendo is definitely still supporting it and will continue until at least January. Splatoon will almost certainly get a sequel for the NX, and its current popularity will likely carry it until that point. I wouldn't be surprised if they end up trying to port Splatoon to a handheld sooner or later, even though I would personally be against that decision. No Splatoon representation in Smash isn't the end of the IP, nor a sign that Nintendo wants it to go away. Heck, part of the reason they weren't included could have been because of the already-existing Mii costumes. It's a new IP, and one of Nintendo's most successful, considering the previous fact. They wouldn't leave it to curl up and die quite yet.
The facts are that Inklings didn't win the ballot, Nintendo made a deal with Square Enix, and wanted to promote the new Fire Emblem game coming out. However, Splatoon is still getting free content. Merchandise is being made, the fanbase is still extremely active even after half a year, the game is evolving because of the competitive community... Overall, Splatoon doesn't seem like it's going any time soon and we have no reason to worry about its death yet. The only reason I could see Splatoon dying would be because of the release of the sequel, or Nintendo dropping Internet support for the WiiU sometime after the release of the NX.