It is true that Switch sales are going to explode during the next few years, but it mostly won't be due to games that create new gamers like Wii Sports and Brain Age.  Switch sales will explode, because it is Iwata's answer to Nintendo's philosophical/internal problems.  

While the Wii was a huge financial success, it wasn't sustainable, because their developers didn't really want to make games for it.  That was the Wii's biggest problem and why Nintendo seemed to stop supporting it so quickly.  On the other hand, their developers wanted to make Gamecube style games, but the mainstream didn't want to buy the Gamecube.  Also if the mainstream doesn't want the console, then third party companies won't support it either.  So that was the Gamecube's main problem.  What Nintendo really needed was a console that would sell games that their developers wanted to make and also still appeal to the mainstream.

Enter Switch.  It's flagship games are the core type of games that their developers are comfortable making like Zelda: Botw, Mario Odyssey and Splatoon 2.  At the same time, Nintendo's handhelds always appeal to the mainstream.  Switch has internal appeal to their developers and external appeal to their customers.  Switch will get lots of third party games, because Nintendo handhelds always get lots of third party games.  In short, Switch uses all of the strength that Nintendo always has in the handheld market and brings that strength to the home market.

That is where the explosion of sales will come from: the home market.  As the Switch's library grows and grows, the home market won't be able to resist coming over to Nintendo.  They will "switch" from Sony to Nintendo.  That was Iwata's plan all along, and I expect it will be successful even though he isn't around anymore to see how it will all play out.

Okay in all honesty, the bolded sound like absolute Bullshit. I seriously doubt Nintendo's developers didn't want to make games for Wii because they hated casual gamers or casual games. If anything, Nintendo's devs enjoy making quirky ideas like those, as evidenced by Labo being a thing. I also don't buy the idea that the Switch is successful because of core gamers alone, when we've seen the contrary. Switch is popular with a wide variety of people, casual and core alike.

If you think Nintendo devs liked making Wii games so much, then why didn't they keep making motion control games?  

Oh I mean a new 2D Mario or Super Mario Maker 2 

I'm not convinced the "casuals are on mobile now" is a suitable argument to why games like Wii Fit or Nintendogs (which would be a great fit for Switch in particular) shouldn't be made or couldn't sell like gangbusters just like 10-12 years ago. This more mass market audience was willing to game on consoles before, and I think they'd do so again.

These people I feel would appreciate the superior hardware and controls of playing on a home or handheld console more than simply tapping on small mobile screens, the same way most that fancy themselves core gamers would. There just hasn't been many great games for this crowd on the console front lately, because most devs don't seem to take them seriously. But these people are out there, and they are willing to play great quality games on quality hardware like the rest of us. They just have a bit of a different style in gaming tastes than most of us.

That mcrowd will pretty likely return. Look at Just Dance. Even after the platform was deserted by any other release the Just Dance base only slowly degraded. So the people were there and waited for games. Bring the games and the people who play them will be back again.

2019 will be Switch's biggest year, just going based off its current known lineup alone.

My guess is that they ran out of ideas. Nintendo has always been a bit of an ADHD company, constantly shifting their focus to different concepts. They figured the Using the Gamepad's motion controls, and the Wii Remote, in conjunction with each-other would be a great idea. When that inevitably failed, they went right back to the split-motion controller concept with the Joy-Con. 

I see what you're trying to argue here regarding Nintendo's mishandling of the Wii's last few years and the Wii U, I just don't buy it. 

Because they were aware that not every game needs or even benefits from them