Those Switch software sales wouldn't be awful actually, but for Splatoon to sell a million, there are going to be significantly more Switches sold than 1.4 million. Games very rarely reach the type of user base penetration that many people are expecting out of Switch software right now. Splatoon on Wii U is basically a best case scenario for attach ratio with for a single game to a piece of hardware, and that isn't even at 0.5 software/hardware ratio.
If Splatoon 2 sells close to a million, I would expect at least 2 million for the Switch. If it reaches 1.5 million, the system will sell closer to 3 million or more.
Splatoon has sold 1:1 ratio with the hardware it was on. It's at 1.5 million physical, majority of that 1.5 million was bought by people who were also buying the Wii U.
Wii U(pre-launch) - 2.337.877, while it's currently sitting at 3.319.808. Gotta keep in mind that Splatoon was only bundled with the Wii U around spring time of 2016. In December of 2015 instead of having a Splatoon bundle for example new Wii U owners were buying SMM Bundle and Splatoon separately. While when the actual bundle was release it was after the console was already receiving it's last meaningful exclusive support(Pokken & StarFox). I also think it was the most expensive bundle on top of everything else. The Wii U only sold 300k in 2016 and probably around 250k of those are the actual Splatoon Bundle.
I'm sorry I should have been clearler - I was referring to hardware ratio post launch. The people who were buying Splatoon were the people buying the Wii U. Splatoon was the main motivation behind an additional 1 million Wii U's being sold. There were a couple of other titles like SMM & Minecraft. But SMM sales stopped after it's bundle was discontinued, it only reappeared after SMR was released. In terms of Minecraft I expect it to be a very popular game on the Switch