Last gen was an anomaly. I don't think we'll ever see sales of that scale, for all platforms, anytime soon. Until economies in other parts of the world (Brazil, China, India, etc.) strengthen more.

Hardware - Software
154.88 -- 815.24 - DS
101.00 -- 918.15 - Wii
82.77 --- 824.99 - PS3
81.28 --- 888.86 - Xbox 360
80.78 --- 295.25 - PSP

500.71 -- 3742.49 - All Platforms

Attach rate is pretty useless. It only goes up because only core gamers are buying your product.

I don't think that's how this work.

DS+PSP HW = 230M
3DS + Vita = 52M

DS+PSP SW = 1B
3DS + Vita SW (considering your digital sales) = 160M

DS+PSP rate = 4.34
3DS+Vita rate = 3.08

Now imagine they have the same attach rate with some guessed numbers

3DS+Vita = HW - 100M
3DS+Vita= SW - 434M

It looks like trouble to me. A drop bigger than 50%.

I don't think it was an anomaly. Las gen was the first time 2 legitimate handhelds were released in the same period, and both saw strong sales. Then mobile phones came along, and handhelds are seeing a regression of sales not seen since 1983. I see no reason why sales wouldn't slow down further next generation. Nintendo and (maybe) Sony need to step up their game big time if dedicated handhelds are gonna survive another generation.

The DS actually has a higher attach rate than the 3DS at the similar point in their life cycles by the way.

Going back and looking at Nintendo's software shipments:

After 13 quarters, the 3DS has shipped 43.33 million hardware units with 162.9 units of software for an attach rate of 3.76.

After 13 quarters, the DS had shipped 64.8 million hardware units with 330.88 units of software for an attach rate of 5.1. 

So the OP's point even on the basis of attach rate has been debunked. Attach rate tends to follow a bell curve, because at the tail end of a life cycle (especially for one as long as the DS) you get a lot of cheapo/budget parents who buy their kid the system with like 1 game that brings the attach rate down.

I suspect Sony will just design mobiles and tablets with Playstation in mind (either through Playstation Now or decent mobile hardware). I don't see them bothering with dedicated handhelds after Vita.

Nintendo, you can never really guess what they'll do!

I  think that Sony will transition completely to a mobile Android handheld, with exclusive games and exclusive backwards-compatibility with the Vita and PSP. 

Nintendo will stay in the dedicated handheld market for at least one more generation, but the way sales are declining, they will need another DS-level success to keep the handheld market floating. 

Nintendo's going to make a hybrid platform that runs the same games (so no console/hh distinction) anymore, I think that's pretty much the worst kept secret in the industry right now.

I'm not sure if playing Android games on a dedicated game handheld makes any sense ... I already have my phone which plays those games, so why would I carry a second device to play the same games on if that's what I was really interested in? And I'm interested in more complex games that require physical controls, odds are I'm not going to be playing Angry Birds on it as that sort of defeats the point. 

See the Xperia Play. It could do PSP games and Android games, plus it had physical controls. Sony's new handheld being a hybrid that can do exclusive new games along with older Vita/PSP games, not to mention the huge library of Android games (most of which suck, but there are a few good ones here and there) is certainly possible. 

Except Sony never did release PSP games on the thing, nor did they update the PSX library.  As much as I love my Xperia Play, it's a dead end.  Sony needs to find a way to make a "great" controller add=on that doesn't make any Xperia line phone ginormous.