Your post reeks of biased and has many flaws...

"And nope, my reasoning is perfectly clear"....

"It's a new IP" Oh yeah so was many games released such as Wii Sports/Brain Age/Captain Toad/Pikmin/Luigi's Mansion/AC/Tomodachi Life/etc. They all failed too right?

"lacking features such as voice chat that games from the mid-2000s already had" Thats why Most Nintendo games that have no online or even if they have online support no voice chat and those games failed too right? Oh Wait!

"not mention the potential low scores due to lack of content that many reviewers will probably give to this title" Premature, Whats a low score anywayz? Under 75? Under 70? If this game gets an 80 or above then what are you going to say?

" the game being already crippled by the poor Wii U user base" Yes the Userbase will effect this. But that doesn't mean it wont do a good amount. Didnt stop games getting 20%/30% or 50%+ attach ratios.

"Just a nice opinion piece with a lot of logic behind it." More like an opinion based on wanting the game to fail :-/

There is a big reason why MK and SSB became big because all the charecters in those games are existing ones and familiar to everyone, but splatooon is is not using a single charecter from their IPs which is a big difference so dont expect success in this game unless they  add charecters from their IPs. Nintendo games sells only if the game has their IP charecters but not from the scratch. Always nitendo finds big success in milking their ips but not new original IPs recently and times have changed so Nintendo and gaming is not like old days for new IPs to get big sucess unless the platform is big, huge marketing, reviews are great for the game.

To be honest, i expect even bigger numbers for witcher 3 on ps4.

Skyrim was 8 Mio on xbox360 and 6 Mio on Ps3.

The hype around witcher 3 is pretty much the same as it was for skyrim in 2011/2012. You cant compare witcher 2 sales to 3. 3 is on a complete new level. Just look at trailers and gameplay on youtube, where almost every single comment is like :"do you need to play witcher 1 and 2 before 3?)

Is that why Brain Age/Nintendogs/Pikmin/Tomodachi/Wii Sports/etc became so big? 

Also i mean aside from Mario (in DIFFERENT GENRES and play completely different from others that are innovative) and maybe pokemon, what Nintendo games are milked?

Star Fox?

Metroid?

F Zero?

Kid Icarus?

Earthbound?

Fire Emblem?

Zelda?

You're ignoring the fact that the game is already charting high on Amazon worldwide (in addition to preorder sites like COMG) in favor of what you think the game should do based on its features (or lack there of). Try to find a new IP that was heavily pushed by Nintendo in the past 30 years that hasn't sold at least 1m and you will be sorely disappointed in the results.

So you ignore Pikmin, Animal Crossing, Tomodachi Life, Wii Sports Resort, etc.

Ok, whatever. ;)

Is that why Pikmin sold well? And Animal Crossing? What about Xenoblade? Tomodachi Life? Or Rhythm Heaven, Kid Icarus, Brain Age, Golden Sun, Endless Ocean, etc. 

I don't want the game to fail, the game is setting itself up for failure, well Nintendo is if we are being technical.

Here I go though:

"Oh yeah so was many games released such as Wii Sports/Brain Age/Captain Toad/Pikmin/Luigi's Mansion/AC/Tomodachi Life/etc. They all failed too right?"

Let's see, while being new IPs Captain Toad and LM can be omitted thanks to being within the Mario Universe with already recognizable characters, Wii Sports was heavily bundled, Brain Age had the casual crowd by the balls and Tomodachi was riding on the Mii's success, Pikmin is your only good example but where is Pikmin 3 now? Can you see Splatoon having any of the traits these games have had? I can't

"Thats why Most Nintendo games that have no online or even if they have online support no voice chat and those games failed too right? Oh Wait!"

Most sucess stories on the Wii U are heavily attached to Mario and pre existing IPs in one way or another. Remember this is a new IP!

"Under 75? Under 70? If this game gets an 80 or above then what are you going to say?"

A nice round 80 score won't provide any legs and that's all I'm going to say, hell even if the game was in the range of 85-89 I doubt the impact would be enough to provide a significant boost to sales after the first week and that's IF the game gets good scores :)

"Yes the Userbase will effect this. But that doesn't mean it wont do a good amount. Didnt stop games getting 20%/30% or 50%+ attach ratios."

Again this is a new IP with neither the help of pre existing Nintendo franchises or Mario, anything can happen to Splatoon and I think we've had enough new IP bombas on the system to justify my statement.

Most of the titles you mentioned are for handheld where Nintendo is dominating so its easier to find success because of huge install base they have and market share and Wii sports is motionware title found success fot its motion features back then but failed with Wii U sports game. Captain toad is a familiar charecter milked in a budget indie game but not a fresh new IP entirely and not a AA scope game. Pikmin game times are very diferent to find success back hen which can proved with Pikmin 3 sales so even if they made that game now it wont sell like 2001 game.

You have to accept that times have chagned and these days new IPs to get success need a platform with install base, good marketing, excellent reviews and content that everyone expects (mostly online), otherwise it wont get success. For me Splatoon can sell average (LTD 500k min - 1m+ max in best case without bundles) as they already got issues in content and price ration so mostly reviews will be 75-80 due to that. 

I wont bother with the others as we'll be here all day.

That said your entire point is invalid here. You practically said the score doesn't matter^ Most games get under an 80, They have legs (ex: Destiny) Bayonetta 2  got 92 on meta, no legs.