Unlike Galaxy 2, Splatoon 2 is launching on a new platform that's far more successful than the one its predecessor was tethered to. Splatoon 1 was held back by being on unpopular hardware, Splatoon 2 won't be.

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. 

We'll see. The fact that Wii at the time was still a heavy seller and didn't manage to garner more sales is more telling I think. But if we're talking sequel territory, let's jump from Galaxy 2 to 3D World... you can't really guarantee anything at this point in time. We don't know how the fanbase is going to react to another Splatoon. There's always the vocal minority to kinda bolster and rally the troops but week 1 for me will be the most telling. However, in retrospect 3 million might have been a bit too low, I understand that much. I'll say 4.5 LT. It really depends on how Switch sells and if Splatoon 1 players see enough incentive to buy the second one. For me, I don't as I see a game running off the exact same engine with a few new perks here and there. And that's just me, I'm sure there are fans who think that's enough to come back for more. Again, I'd love to be wrong, I hope I'm wrong but at this point I just don't see the appeal for Splatoon 2. Japan disagrees whole heartedly of course but yeah, that's where I stand.

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.

I'll take it one step further.
Splatoon 2 will reach around 14-15 million - Pokemon levels.
Super Mario Odyssey will be around 12 million.
Breath of the Wild, it will take some time, but I do believe it will crack that 10 million mark and perhaps even go for 11.
Other games that I think will get there are Pokemon (effortlessly), Animal Crossing Switch, and maybe even Mario Kart 8 Deluxe.

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?

Mario odyssey probably will. I expect BotW to get close but ultimately fall short. Splatoon exceeded our expectations on the WiiU. I expect that to continue and for it to grow and thrive on the Switch. Probably near double the numbers and hit 5-6 million.

Splatoon 1 on the Wii U is at 4.8 million as of the end of March.  https://www.nintendo.co.jp/ir/en/finance/software/wiiu.html 
So, it pretty much already hit that 5-6 million mark on the Wii U.
If it does double that amount, than we're looking at 10-12 million for 2 on the Switch.

I guess Splatoon is the least likely to hit 10mm.  So, if they bundle it with the system, it would significantly increase the odds of this happening.  I don't think 10mm is a guarantee for Zelda either though.  Odyessy will probably hit it, assuming it is a decent game, and switch sells at least 30mm lifetime.  If its a great game, it could hit 10mm even if Switch only sells 20mm.  

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.