MS and Sony gain nothing from people purchasing 360's and PS3's. Those are dead systems and they would much rather potential customers pick up an XB1/PS4.
They gain free profit.
Let's say just as an example both are $99 and they make $50 profit for each sale, and lets say each sale buys a game so per console that is $75 profit.
$75 x 1 mill is 75 mill, and this isn't including those who buy accessories, more games, or/and buy xbl. So that could be over 100 million dollars in cash.
I mean that's free money.