The PS2 has several advantages that the PS4 does not in terms of extending sales into the succeeding generation.
1). Price. PS2 was $129 in the US when the $499/$599 PS3 launched.
2). Market dynamics. PS2 enjoyed an astounding market share with very little global competition from other devices.
3). New markets. PS2 had the benefit of gaining traction in markets that had not yet really gained mainstream video game access.
4). Counting. PS2 production was how sales were tabulated by Sony back then. Not units sold to retailers. So any warranty replacements, retail demo units, developer kits, etc...were counted as part of that 157 million. Sony changed this practice to the industry standard in 2007 (I forgot which quarter).
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