"The first one was a new IP, and it needed word of mouth to build a fanbase"
And it built that fanbase on the least popular console that Nintendo ever created. Is it your contention that a game's sale potential is *not* in large part a function of the install base it is released on?
"the sales were more... "ever green", or, long term."
The term 'Ever green' titles specifically came into use by Nintendo to describe those games on the Wii whose sales were directly linked to the monthly hardware sales because they were basically complementary goods. Someone bought a Wii, regardless of year they also picked up Zelda, Wii Play, Wii Fit, and Mario. Consequently, the lifetime sales of those franchise iterations correlated strongly with the yearly sales and ultimately the install base.
Likewise, because the Wii U's hardware sales were dire (both in monthly terms, annually, and lifetime), pretty much every single franchise Nintendo released sold worse on it than entries of the same IPs on their other, more commercially-successful products.
"This one will be more frontloaded (and it's perfectly normal)."
While you have your crystal ball out, can I get some lottery numbers?
"The only way we could have seen it with the first week is if it would be the third game with the second on Wii U too"
Not really. When you have a game system that is tracking way ahead of its predecessor and a game that is tracking way ahead of its predecessor, attributing the latter to the former is not much of a stretch. If Switch sales continue apace and Spla2n sales suddenly grind to a complete halt, then we can re-assess.
"It's just too early to say."
I disagree. The scientific method is not 'Don't ever come to any conclusions until you have perfect data'; it is 'Draw the conclusion you can on the best data available, and revise if/when better data becomes available'.