The replies on the first page of this thread are very funny. I kind of wish I was on this forum back when this was posted. I would have been a lot more bullish than 10m for Zelda. The best I can do is this thread from March 2018.
It was certainly underestimated I even had a prediction of 9m or so sales and that was seen as optimistic in comparison mainly due to people not fully realizing the change in role BOTW has initiated for Zelda, the franchise has gone from being the dedicated first party that is handled like a Triple A to now filling the GTA/RDR void in the first party line up it has specific role in the platform's library and momentum now. Even if someone hasn't been into the franchise before BOTW acts as a great alternative to the R* games which puts it into a powerful position in the library as now it can potentially rival or even as seen here surpass Mario in selling power as waiting for the next mainline Zelda game will possibly be equivalent to waiting for the next GTA/RDR now.