But isn't the argument here to compete with Apple and Google? So MS would benefit in doubling their brand strength and getting stronger EU and especially Asian presence, along with Sony's 1st parties. Access to Sony's media isn't such a big thing next gen (Sony dropping the ball on that one). I'd say both would benefit, at least regards strength of the next-gen console versus other rivals. Whether they'd benefit fiscally, which is all that matters to them, I don't know.So a cooperation effort would mostly help Sony, and not MS all that much, which is why I think MS would never go for it even if Sony tried to convince them (which I think they're too boneheaded to do anyhow.)
Edit: If we look at this gen and they had gone halves on a PlayStationX, they'd have a single platform that'd be easier for devs and possibly a larger install base because people weren't having to choose between one or other. As PS3 vs. XB360 is nearly 50:50, getting an equal split of the profits of PlayStationX this gen would have been about right. Excepting of course it doesn't work that way, and a company may have a different capacity to turn a profit given an install base (see Nintendo). There's also the issue of arguments wtih them wanting to take different approaches - the main reason in life to avoid any cooperation whatsoever!