There are arguments about every possible position for a title, that's what people do, they debate. That someone made a position on something says nothing. It is absolutely not true that Mario Kart 8 performing poorly on Switch was a "common belief" by the time it was actually coming out.
I think it was a common belief up until the we saw the trend in terms of pre-orders and anecdotal reports from retailers. I think before Switch actually released majority of people expected it to sell worse or equal to MK8 in Japan, only 6 people in the prediction league for example predicted it to sell more than 1 million this CY. Which to me always seemed to underestimate demand for a handheld MK in Japan. Compare LTD sales of MK8 and MK7 and it's easy to say that MK8 did poorly relative to MK7 in Japan.
It wouldn't be surprising for example for MK8D to sell twice or three times the amount MK8 did LTD in Japan, especially if MK9 isn't announced in the next two years. Hell if Nintendo really wants to push the title they could always announce a Splatoon themed Cup and more battle stages for next spring. I believe MK8D is already well above 4 million sales WW and what better way for Nintendo to reengage the audience right before they release their paid subscription service next year.
A lot of the skepticism around MK8D and BotW I think was because of skepticism on GAF around the Switch. There were countless of posts talking about "no games" "just ports" "I already played this on my Wii U" "Switch is not a handheld" "Switch is too expensive to be a handheld" etc. Personally I addressed that back during the MK8D release week:
People who doubted MK8D, in a way doubted the system itself. To majority of handheld owners this is a game thats following MK7, same with Minecraft in a couple of weeks. These games are simply making their way to a portable and due to small audience of the Wii U it will also be their first MK8 experience for the majority of the owners especially in Japan. Switch as a device is simply far better than either the Wii U and 3DS and the launch lineup is miles better than either system.
In terms of MK8D - Battle Mode is actually something that will probably make MK8D a far greater game than the original especially on a device like the Switch. The portability and instant local/online is simply far greater strength than either Wii U or 3DS had at launch. We are talking a full class of students being able to play or enjoy something like MK8D without everyone having to have brought a Switch. The Switch is no doubt going to be one of the most popular gifts this year for kids.
The only reason MK8D isn't selling at the same speed as MK7 is because of the shortages Switch has faced since release - MK7 sold 1 million during it's first month. Granted that was a holiday and 3DS was selling 1.6 million during that same time-frame. If Switch is readily available this December(>1million) I think we'd see MK8D as the 2nd or 3rd most bought game physically on the system. My personal expectations for December in terms of Switch titles does depend on whether SMO bundle is the only way to buy a Switch this December. I could certainly see SMO selling >1000K if it's the only way to purchase a Switch in December and this to depress other title's performance. I also think we'd get physical Minecraft BT version in/before December
Without SMO bundle:
1) Splatoon 2 >850K
2) Minecraft* > 600K
3) MK8D > 500K
4) Super Mario Odyssey >450K
5) Zelda:BotW > 250K
With SMO bundle:
1) Splatoon 2 > 850K
2) Minecraft* > 500K
3) MK8D > 350K
4) Zelda:BotW > 150K
5) Arms > 100K
