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.
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.
I have other comparisons that fit the bill...there's plenty of them. Tons of games that have sequels that release on new hardware that sells less than the original. Just one game, to note Zelda: OoT and Zelda: Wind Waker. And there's games that go against that. So calling that type of comparison..bad is kinda biased. Both scenarios exist. The issue I have with Splatoon 2 and believing it'll sell past the original is that it does little to really cause me to say "wow" this is much better than the original, I have to have it" For me, I don't have that lust for Splatoon 2. There's not enough difference to make me want to get a sequel. And again, that's just me. Shooters, that's where they make their bread n' butter. Making something that works and something addictive that people will want to come back to no matter how similar the sequel is. That is a very common phenomenon, Call of Duty just to name one. I'd love to see it surpass the original, that's more cash in Nintendo's pocket and it's healthy for the entire gaming industry. There's just too many variables at this point to come up with an good conclusion. I'd have to see FW combined sales but that takes the wind out of prediction all together doesn't it?