The lifespan of the PS2 isn't unusual, but the sales level it is currently at is somewhat unusual compared to the previous generation; and the reason for this is simple ... Last generation the combination of the PS2's full backwards compatibility with the Playstation along with its first price reduction to $199 in 2002 meant that most people who were looking for a Playstation from mid 2002 and beyond bought a PS2 instead; in contrast the high price of the PS3 and the lack of backwards compatibility has prevented people from being upsold to the PS3.
Now, there is no way to how many potential sales the PS3 "Lost" to the PS2 but (being that the PS2 has sold at a similar rate to the PS3 since the PS3 launched) it is possible that the PS3 could have seen 10+ Million more units sold by now had it been more affordable and had full backwards compatibility.
