The lifespan lasts as long as it still sells signifficant software.
The Xbox does still sell software, but it's rarely within 5% of the total of the current gen consoles.
Its lifespan is over, it is merely having some death-cramps.
The Ps2 though, is still selling. As long as you sell within 33% of the losing console (and the consoles are pretty even), its lifespan is still there.
If it sells below that, it becomes relatively irrelevant. 20 or 25% might be a better number...
Of course, this definition won't always work, but it gives some relevance to it.
The NES didn't last 20 years just because it shipped 12K in 2001.
The GBAs lifespan is also over, but it was still going in 07.
Software sales make the console, not hardware sales.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS
