Well everything is possible, Splatoon 1 sold 1.7m on 3.3m Wii U consoles in Japan, that's more than 50% attach rate, and Switch will most likely end up selling much more than Wii U sold. I think Splatoon2 will be 3m+ game in Japan, so even 5m is possible.
Except massive attach rates are sort of common on underperforming consoles.* Actually, I just made that up, now I'll have to go check... :) But seriously, Super Mario Galaxy didn't magically sell 25 million just because it was on a much better-selling console than Super Mario 64.
I think 5 million in Japan alone is way out there, but I do hope for Splatoon 2 to have a much better showing worldwide thanks to Switch's improved sales. I'm thinking more like 10 million, though.
*Hmm, checked a few series like Mario, Halo, GTA and it's kind of inconclusive. If we want to take a pretty optimistic Mario Kart type scenario for Splatoon, then 3.5-4 million might actually be feasible for Japan, that would be nice to see. I'd be shocked to see >4 million.
Of Course, I don't say it will again have same 50% attach rate, but Its not so simple in any case, M64 was first 3D Mario and was massively popular, it was launch game and it was best selling N64 game, despite being great game and having great sales, Mario Galaxy didn't exactly had same impact like M64.
And Splatoon is totally different story, its biggest new IP in around last 10 years in Japan, popularity is still huge (just look pre orders in Japan, it will probably sell around 500k 1st week if Nintendo have enuf copies of game), Splatoon 1 sold 1.7m on failed Wii U that had only 3.3m sold units, and now we have sequel of highly popular game that will have much bigger instal base and actualy on handheld (Switch is in same time handheld and thats alone game changer in Japan), so like I wrote, 3-4m is almost certain, so even 5m is possible.
If it does it would absolutely epic, it would be already epic selling 2 milions.
Well Splatoon 1 sold 1.7m on 13.5m Wii Us, so 2m wouldn't be exactly epic. :)
No chance whatsoever.
It is launching on a console with an install base of less than 5 million. Even if it sells very well for the rest of this year, and reasonably well through 2018, and still sells okay through 2019, *and* the Switch continues to sell very well, Splatoon 2 *may* get to 10mm worldwide. That gives you 2-3mm in Japan. Even that requires a whole lot of factors to fall in the game.s favor.
Splatoon 1 sold 145k first week on around 1.7m Wii Us when was launched, and ending selling around 1.7m on around 3.3m Wii Us at end, Splatoon 1 on Wii U was around whole year constantly in top 10 best selling games in Japan on weekly basis, in other words it's type of game that will is selling great in years. Splatoon now has incredible popularity in Japan (preorders are incredible strong), Splatoon 2 despite it will be launched on smaller instal base than Splatoon 1, can sell around 2-3x better first week than Splatoon 1 did (its posible that can sell around 500k if Nintendo has enuf copies of game), also Switch will have much bigger instale base than Wii U had, and in same time its handheld (that alone is game changer in Japan), in other words, it will easily pass 3m in Japan, and even 5m is possible.
It's a year 1 title and will sell for years. Anything under 5m and I'll be dissapointed.
You do realise that much less than 10 games ever achieved to sell more than 5m in Japan, and that most of them are GB and DS games!? And you will be disappointed if Splatoon sell less than 5m!? That's hilarious. :D