I don't think Microsoft and Sony think that way. Look at the extraordinary lengths Microsoft and Sony went to in the previous generation to provide back compat -- Microsoft wrote an amazing Xbox emulator, Sony build a whole expensive mini version of the PS2 into early models of the PS3.
And this generation it seems that Sony is trying to provide PS3 emulation using their cloud gaming system.
Some of the benefits of back compat to platform holders:
1) Encourages customers to upgrade sooner and to choose the compatible platform.
2) Increases the roster of games during the first few years of the new console's life.
3) Extends sales of the previous generation hardware and software. (Software because it can be played on both old and new hardware, and hardware because there's more new software available.)