I chose galaxy because it was a huge Mario game on a platform with well over 50 million units under its belt at the time. And there's no way Splatoon is as highly regarded as a selling power house than Mario's 3D outings or even his 2D ones. If Mario 3D World was released on Switch with upgrades, I'd expect it to sell less. It's too similar to the previous game which to me at least, means lost interest for the sequel. I'd love to be wrong and I probably will be but this is what I think. Calling something a sequel and it looks like a 1:1 ratio with added content and same engine...that really doesn't rub me the right way. That would mean MK8 deluxe qualifies as a sequel which isn't true. This could go either way, there's no way to gauge if Splatoon will continue its momentum as a series. That's my prediction, don't throw stones at me just yet.
I think your comparision is a bad one becaues Super Mario Galaxy 2 released on the same platform as Super Mario galaxy. The sales potential of Super Mario Galaxy was probably already reached, and the only way to go was down. Since it was a single player game which didn't inovate, but rather just expanded the original, it makes sense that Super Mario Galaxy 2 failed to sell as much as Super Mario Galaxy.
There are a few differences with Splatoon.
1. The Wii U's userbase was small which limited Splatoon's sale potential.
2. It is a multiplayer game, which means it doesn't have to break the theme and formula to sell spectacturaly (see: Super Smash Bros, Mario Kart, etc.)
3. The Switch is different enough from the Wii U so that the experience will be different enough. Local multiplayer in Splatoon can be big, and I see this bolstering Japan's interest.
4. Splatoon 2 is releasing early on in the Switch's life span, and has at the very least five years to bolster its sales.
While I don't think it will sell 10 million, like curl-6 suspects, it will definitely sell more than Splatoon.