It's not at all. Most people are predicting huge success from splatoon 2. Even predicting it to become as big as the likes of monster hunter.
You're predictions have just went so far way overboard with almost everything regarding switch that it makes even optimistic predictions look conservative.
A lot of people predicted sub 3 million Switches sold in Japan and there were tons of predictions of Splatoon doing sub 2 million. Any of these predictions are more outlandish than mine.
My prediction of over 4.5 million Switches with 4 million Splatoon 2 copies are in line with my expectations of the game ending up as one of the top 10 best selling games in Japan of all time.
It's the game Nintendo needs to do well in order for it's sub system to have an incentive for Japanese consumers to pay their monthly/annual sub.
For example you say 6 million for Splatoon 2's lifetime is outlandish - yet it's just over 3 times the amount Splatoon sold on the Wii U. Splatoon on the Wii U had a lot of features that were missing that will get corrected by the sequel while it's also a game that will continue to get content like maps, gear, weapons, events, modes.
Main factors for my prediction:
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First portable Splatoon(potential audience of a portable version is far larger than the potential audience on the Wii U)
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Spectator mode(you need a second Switch and you'd be able to record all your matches from a spectator pov - which will lead to more viral marketing for the game)
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New Mobile App integration for streaming/recording(even now if you go to niconico and youtube you could see thousands of videos for スプラトゥーン & スプラトゥーン2 each month even as you need a PC which is not exactly common item Japanese have)
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eSport(Nintendo is pushing the game as an eSport and it will be the first eSport game on a portable device)
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On-going Support(the first Splatoon was supported for an year and that contributed to it's long legs)
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New Online Sub(it's the title that Nintendo needs to do well in order to push subs in Japan in the Winter Holidays when the Switch will be at the height of it's popularity)
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Biggest new Kids franchise(there was a reason why Splatoon did so well in December of 2015 it's an extremly popular game among kids - popular enough for them to buy a Wii U which should not be underestimated)
Lesser factors:
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Additional PvE Mode(new mode that allows new players not to get disheartened by Ranked modes)
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Additional campaign(after the popularity of Calie and Marie the new lore will get more people on board)
Below are some trends that lead me to believe this would be the biggest game on the Switch in it's first 2 years on the market:
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Japanese Twitter(スプラトゥーン2 trending constantly when Switch was announced, trending when the testfire was announced, trending during last direct)
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スプラトゥーン on Youtube(each month you have content creators that generate millions of views 2 years after the game was released with no easily integrated streaming option on the Wii U)
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COMG Preorders(before pre-orders are even official)
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Google Trends(スプラトゥーン being searched as much as ドラゴンクエスト in the last 90 days with the testfire leading to searches for Splatoon to really blow up)