This game is going to die, at least in America, and it may well be deserved. I can't see the competitive community or even the casual playerbase surviving into next year. The reasons are numerous.
Toxic community, to start. There seems to be three distinct groups that make up a lot of Splatoon's actively chatting fanbase: hyperactive kids, immature adults (that'd be me), and a particularly disgusting third category I won't mention but you can probably guess. ****-talking and talking behind other's backs abounds, people throw fits publicly and privately, no real penalty in-game for inappropriate behavior... the list goes on and on. The only reason this game doesn't have people cussing at each other and randos throwing slurs at other randos is a lack of in-game communication - Splatoon's fanbase seems less vulgar than some but it's JUST as nasty.
On the competitive side of things, teams are disbanding all over and notable players are quitting. Japan and EU don't seem to have nearly as many personality problems as we do, and their skill level reflects it. The skill gap is only going to widen as teams like Spicy Kraken Rolls and Quantum keep falling apart with little in the way of replacement talent. How is this sustainable? This game will certainly never be an Esport, that's for sure.
Challenges facing the casual community are a different matter. For one, there's competition. Fortnite, one of the biggest games of the time, hit Switch today and Smash is on the way. People only have so much time and we are going to lose a great many casual fans to games that are newer and less flawed. And then there's paid online... no matter what Nintendo does, that will wound the playerbase as people refuse to pay no matter what for a multitude of reasons. And if Nintendo does nothing and demands payment for peer-to-peer 16hz connections, that'll cause even more dissatisfaction and departures. Between those two factors, there is NO way the English-speaking playerbase isn't going to shrink. And no online game, regardless of what it is, can survive without casual players. Lose the casuals and the game's relegated to only the most hardcore who won't exactly be welcoming.
And the game itself is stagnant. There hasn't been a patch in well over a month, and the last few patches have done little to change the trajectory of the meta. New weapons and stages promise to change things up, but ultimately the only thing that's changed is player perception. While some think it's good for the meta to evolve organically like this, it also means outstanding issues with the game remain unfixed. Kinda comes off as bread and circuses to me.
Familiarity breeds contempt, I suppose. At least I'm posting this doom and gloom in the right place, hah. I fully expect to get some derision for this, but that's to be expected. Do as you will, thanks for reading, press F to pay respects!