More reasons:
-Sony probably won't use the CELL chip the next time around b/c it has given some headaches, but i'm sure that they will use something to make sure that they can make the PS4 BC. BC will help boost total sales and Sony will appreciate the $$$ as they didn't have the advantage of BC this generation, but this is iffy as you can't always predict the way technology will go...
-The PS4 won't have the complicated development due to the CELL, which will save both sony and developers $$$$$$
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