This question basically comes down to.......

1. Can Nintendo make one or two 20M+ sellers a year for 5-10 years?

and

2. Is it possible to sell 20M on a dedicated handheld in the era of smartphones and tablets?

Many people might answer no to the second question. To those people I would ask...

3. Can Minecraft sell 20M on home consoles when everyone has a smartphone/tablet/PC/netbook/laptop that can easily play Minecraft?

If the answer to question 3 is yes, then the answer to q2 is yes. That leaves q1.

Can Nintendo get Nintendo-lucky again?

It's been pointed out many times that Nintendo wasn't lucky. Many choose to ignore the reasons even though Nintendo spent years screaming from the rooftops and explaining in detail how they planned to successfully disrupt the gaming industry. They even invited others to join them, and Ubisoft managed to successfully disrupt Activision in the music genre while Nintendo smashed EA's sports crown, without even trying.

So it all this boils down to not can but will.

Will Nintendo ever read the 'Innovators Dilemna' again? Will Iwata 2017 listen to Iwata 2007? Will Reggie '17 listen to Reggie '07 or is Candy Crush and Flappy Bird the best that normal people can ever expect to play on a £500 non-dedicated device?

Who knows? Nintendo could very easily strike gold again. They could just as easily pull another Wii U.

Can they? Yup
Will they? We'll have to wait & see

The didn't 'strike gold'.

Unless you mean that they surveyed the land, took samples and knew gold was there with over 80% certainty.

I don't think any company will reach Wii+DS heights again, let alone Nintendo.

How does the fact that the answer to q3 is yes make the answer to q2 also yes?

Doubt it... Specially DS levels but who knows, maybe they can find the magical fairy that made those two a massive success once again but it will be extremely rare

DS 'only' got 4 20 million+ sellers, so no, unless it's a Wii Sports style mass-bundling, they can't sell 20 million+ copies of one game on handhelds anymore, because they'll never have a handheld tat rivals the DS's success.

This is exactly what I meant.

I watched them do it

Probably nope.

Numpty.

It was those games that made the DS a success, not the other way round. What you're essentially saying is

'they can't make software that sells 20M because they'll never make a 20M system seller'