I don't see why a similar growth from 7th to 8th gen as from 6th to 7th gen should be possible. At least in terms of consoles, as I assume that is what we're speaking about.
A middle-of-the-road estimate of the sales of this generation's consoles seems to be somewhere around
Ps3: 75 million
X360: 75 million
Wii: 120 million
That puts the 7th generation at 270 million or so. That's a roughly 35% increase over 6th generation's around 200 million sold. That's quite close to the increase that 6th gen saw over 5th gen, which was at 35-40% as well. And the 5th gen saw a roughly 100% increase over 4th gen. The 4th gen "only" saw a 25% or so increase over 3rd gen, though. And 3rd gen saw a ludicrously large increase over the ones before it.
All in all, a 35% increase isn't that major. The 7th generation didn't see a huge increase over the 6th generation. The reason for that is most likely that the financial crisis hit at the time it did. But shouldn't then - logically - that increase come when the economical situation betters? I'd say that seems likely.
I don't see any real reasons as to why the 8th gen shouldn't be able to see 350 million or more sales. This industry has always grown, I don't see why that should stop now.
As for the handhelds, we're looking at 270 million or so for DSPsP. That's quite a huge increase over the 80-90 million of the GBA. Now, we won't see similar increases next generation, but there's no reason the 3DS shouldn't have the potential to sell 200 million. As for whether the handhelds will increase, that depends a bit on the next Sony handheld. If there is none, which currently seems possible, or it fails massively, we might see no growth or even a slight decrease.
There's no real reason as to why next gen won't be even bigger than this one for the consoles, as far as I can see.
