This is mostly from memory, but from what I've seen on the Nintendo Twitter, Animal Crossing is one of the most successful series. I doubt they'd drop it. Animal Crossing main series games generally take many years to develop, looking at the time line for series releases will demonstrate this pretty quickly. The spin-off games came out partially because the developers wanted to demonstrate some of the new mechanics they've been working on (HHD) and in the case of the amiibo game "we just wanted to make animal crossing amiibos", the game was more of an after-thought. I think they're just trying to keep the fans interested and attempting to make a bit more profit (nothing to hold against them, they're a company, after all!)
I wouldn't worry much about Nintendo abandoning the series! We'll certainly see new releases, it'll just take a while! But the longer they take just means they're taking more time to make the game great for us players! The cell-phone game is supposed to come out soon (sometime this summer, I believe..?) So there's something to look forward to short-term. The cell game probably will be an add-on app rather than a stand-alone game, but we're sure to see new features added to our beloved game!
Happy gaming everyone <3