Saying that any series that hasn't passed 10 million in a single entry is "niche" is ridiculous, 99+% of franchises never cross that mark.

Even if we exclude remasters/ports, Zelda has already passed the 7 million mark twice, and Breath of the Wild is going to pass 10 million anyway, it's over a third of the way there as of its first month.

If BOTW passes 10 million then it will no longer be niche - I doubt it will pass 10 million, I expect around 7-8 million. But if it does pass 10 we'll call it mainstream. Until it does though its conjecture. 

I think this whole debate rests on the fact that you're using your own version of the word niche which in reality doesn't fit any of the franchises mentionned. The best definition of niche that I found is in French but I'll translate it best I can to English : 

Un marché de niche est un marché très étroit correspondant à un produit ou service très spécialisé. Le fait de viser un marché de niche permet souvent d’être confronté à une concurrence moins forte, mais les volumes de ventes potentiels sont naturellement plus faibles.

In English: A niche market is a very narrow market that corresponds to a very specialized product or service. Targetting a niche market often allows to face weaker competition but the potential volume of sales in naturally smaller.

Zelda, Uncharted, God Of War, Horizon, any franchise that sells multiple million copies doesn't fit this definition. I guess your intention is to differenciate these series with the franchises that sells over 10m copies like CoD, Mario Kart and GTA and I agree with you that there should be two different terms for each category but "niche" is not the term you're looking for.

Whether its multi platform or not is irrelevant to the discussion of whether it is niche or not. Multi platform games obviously have a much better chance of becoming mainstream since they reach a broader audience.   Zelda cant reach a multi platform audience so it is at a disadvantage, but we dont measure mainstream or niche by the size of the platform, we measure it by the size of the market. Zelda is niche by that measure. You cant be mass market selling 4-7 million units when GTA 5 sells 80, mario kart 7 30 and even Witcher 3 selling 10+ when it's an almost identical game. Uncharted is niche - it's a great franchise, but it gets crushed by real mainstream franchises like COD and GTA in terms of sales. again, its disadvantaged by being on one platform but that' the way it is. You need to hit about 10 million to be mainstream now.

Those hit 10 million due to remakes, not indvidual buyers. heyre around 8 million individually probably.  Similar games in similar genres have sold far more - Witcher 3, Skyrim, Fallout, RDR and Far Cry. Zelda BOTW is an open world game similar to those (perhaps less Far cry) and I doubt it will outsell them. 

If BOTW passes 10 million then it will no longer be niche - I doubt it will pass 10 million, I expect around 7-8 million. But if it does pass 10 we'll call it mainstream. Until it does though its conjecture. 

I think this whole debate rests on the fact that you're using your own version of the word niche which in reality doesn't fit any of the franchises mentionned. The best definition of niche that I found is in French but I'll translate it best I can to English : 

Un marché de niche est un marché très étroit correspondant à un produit ou service très spécialisé. Le fait de viser un marché de niche permet souvent d’être confronté à une concurrence moins forte, mais les volumes de ventes potentiels sont naturellement plus faibles.

In English: A niche market is a very narrow market that corresponds to a very specialized product or service. Targetting a niche market often allows to face weaker competition but the potential volume of sales in naturally smaller.

Zelda, Uncharted, God Of War, Horizon, any franchise that sells multiple million copies doesn't fit this definition. I guess your intention is to differenciate these series with the franchises that sells over 10m copies like CoD, Mario Kart and GTA and I agree with you that there should be two different terms for each category but "niche" is not the term you're looking for.

That's absurd though because by that logic there are only a handful of "mainstream" franchises in the industry. Your 10 million number is not only completely arbitrary, but excludes many franchises that fit the definition of mainstream in both popularity and recognition.

And Botw sold 3.8 million in its first month, has incredible word of mouth, is one of the best reviewed games of all time, and is the killer app for the Switch, why would it not pass 10 million?