They already shut down Mario Maker's online and that was by far the most popular Wii U game for online (followed by Splatoon and Mario Kart 8)
It was only a matter of time until the rest followed. And I fully expect them to shut down both the Wii U and 3DS online at the same time. So it doesn't exactly come as a surprise. Can't say for sure if it'll be this year but that's my guess. They have discontinued consoles and games for both platforms and since both have free online, Nintendo are only losing money on it. Sad to say it but Nintendo have become just as cash grabby as every other big publisher, if it's not making them money, why would they keep it?
I think the main reason why it isn't quite as likely: The only reason Wii and DS online shut down as soon as it did is because of the GameSpy shutdown, so its timing can't really be used as precedent.- As far as I know, other consoles haven't quite shut down online for most games yet.
Mario Maker is a bit of an odd one, because it's been succeeded by SMM2, and its online server costs, I'd imagine, are
much more than most other games, which mostly amount to peer-to-peer matchmaking.
I can see why they'd shut it down... but they didn't even shut it down completely! It's pretty much in a "read only" mode where you can play any levels that have already been uploaded.