Galaxy to 3D World isn't really a good comparison as that's going from a successful console to an unsuccessful one, the opposite of Splatoon.
Splatoon will be more analogous to Smash Bros, which grew from 5-7 million on N64 and Gamecube to 13 million on Wii.
Mkay, how do you fancy Metroid Prime? And even then that's a cheap shot because that's the only dead and gone era we have of an uncessessful Nintendo platform that we can compare to a successor (Wii). Can't really compare Switch to Wii U LT right now so its even harder to gauge. There simply isn't enough data to make an educated guess. My point is, we'll never know when and to what extent fans will get turned off by a franchise. I think the Splatoon fanbase will respond well to this game but not as well as everyone is expecting it to.
As a casual-friendly multiplayer game that presents a Nintendo take on a popular genre, Splatoon is more similar to Smash Bros than Prime and I expect sales trends to reflect that.
Splatoon has thus far been handicapped by only being available on a system nobody wanted. And yet it still managed to sell about 5 million. That shows just how strong an IP it is. And now that handicap is about to be removed, as Splatoon 2 will be launching on a desirable system. If it can do 5 million when tethered to an unappealing failed system, 10 million on a system that's selling like hotcakes seems quite probable to me.