The Wii. I don't really picture any scenario where Switch will sell more than it. Kimishima's sales prediction for Switch (which is same level as Wii, if I am not mistaken) is overly optimistic but extremely improbable.

Just want to add that 2 of Nintendo's 4 handhelds have broken 100 million units sold- The original Gameboy and the DS. The worst selling handheld was the 3DS which is still over 60 million. 100 million sales for a Nintendo handheld is not unrealistic.

The GBA could've gotten their too had the PSP and DS coming out in 2004, 3 years into its life, cut its legs from underneath.

I do not see the Switch outselling the Wii, but I think it will do really well (not sure how many units LTD though).

The original gb and ds came out in a different time. Mobile gaming is extremely popular. No handheld is going to sell a 100 million again

Still GBA would've likely sold over 100 million fairly easily itself had it not been prematurely pushed off the market. 

That said, yes the portable market has obviously changed. But Switch is a different kind of product too and is reaching demographics the GBA could not. 

There are also things Nintendo could do with the Switch hardware ecosystem IMO that compound sales, like I do think there will be a mid-gen Switch refresh around 2020 (current Switch will then be redesigned on 16nm eventually and aimed more at kids eventually, newer Switch model will continue to serve the lucrative higher end market at $300). 

Most likely not.
Beating the 3DS is hard enough!

It's less realistic for a relatively expensive, not very portable handheld competing in a post smart device age. However novel and appealing the hybrid aspect proves to be, it's not going to be the same craze that propelled previous consoles past that mark.

I'm actually pretty bullish on its chances against the 3DS. Switch has got what it takes in Japan, and it's kicking the 3DS's ass in the West. 3DS took 20 weeks (and a price cut) to clear 900k sales in the US. Switch did it in its first month.

I think Switch has crossover appeal to the Playstation/XBox audience though that past Nintendo portables did not. If Nintendo can keep the hardware relatively high end throughout it's lifecycle IMO they have found (for them) the perfect way to co-exist.

A lot of XBox/PS fans I've shown the machine to are very enthusiastic about it, much moreso than they were to3DS or Vita.

Switch in a lot of ways is Nintendo's answer for how to co-exist in the game industry with Sony/MS I think, but they had to go through the painful process of watching their discreet home console and portable lines have to be forced to merge and to have the Wii brand collapse entirely.