What the DS did was just amazing. 30 million back-to-back in 2007 and 2008 is a far cry above what any other machine has done. Selling more than 20 million every year for 5 years is also quite fantastic.
No other machine has managed over 25 million. The Ps1 managed over 20 million one year (1998), the Ps2 managed it two years (2003, 2004), the GBA might have (it sold slightly 43 million, with just 1.5 years on the market before the DS launched. It obviously sold some later, so it's hard to say for certain), and the Wii did over 20 million in 2007 and 2008.
The 3DS isn't going anywhere near 30 million a year. It's not looking to pass 20 million this year, which means it's not even matching the DS' 5th best year.
The 3DS has sold 6 million so far this year. If it matches the DS' 2007/2008 (peak years) September-January, it'll sell roughly 21 million. It's not going to suddenly turn around and do that, though. The DS had sold 15 million in the first 8 months of 2007. That's nearly triple of what the 3DS is doing.
Counting its full years, the DS did:
11 million > 21 million > 29.5 million > 29.7 million > 27.6 million > 21 million > Successor released
The 3DS is looking to be going roughly (counting 2011, as all it lacked was 2 months from being a full year. The DS 11 million year lacked 3 months of Europe, too)
13 million > 14 million > 17 million
This isn't at all the same kind of development or trending. The 3DS isn't seeing the insane explosion the DS did. No machine ever has - not the Wii, nor the Ps2. The 3DS is following the standard development, much more ala for instance the Ps3.
8 million > 10 million > 13 million > 14 million > 15 million > 13 million > 10 million > successor released
It's hugely unlikely that something magical is going to happen to the 3DS to make it see a DS explosion. Nintendo might make some good choices and have it sell over 20 million in years 4 and 5, but it's not going to go much over that. A
13 > 14 > 17 > 20 > 17 > 14 > Successor released
route is much more likely. That would lead to sales of slightly over 100 million by the time it stops selling.
Of course, something could happen that makes it have an extend life of an extra year (ala what the X360 and Ps3 did), but I honestly think the opposite is more likely. Sub-100 million due to a short life (ala what happened to the Gameboy Advance) is more likely that over 110 million due to an extended one.
