Back when Apple was going bankrupt, Microsoft was on the edge of a Monopoly and decided to save Apple in order to keep competition and I don't think they expected Apple to get as big as it has.
Now I am not sure if the Ouya or even the Wii U can be considered competition for the Xbox and Microsoft, if Sony went under would Microsoft help them out like they did apple? Nintendo is on the brink of collapse too but they are holding on.
A little off topic maybe but hey.
I'm pretty sure the whole console war to Microsoft and Sony is just a big self running advertising/marketing system. Think about it, you have people fighting to the death (Not quite) whilst screaming praises of their own console and ignoring/talking shit about the other. It makes a lot of noise and gets a lot of attention with very little effort on Microsoft and Sony's part.
I think Microsoft or possibly even the Japanese government might step in, Sony is one of Japan's biggest/well known companies and it would be a shamefurl dispray if they went under. Never underestimate the lengths they will go to to save face <.<
Either way, Competition is good and the more of it there is the better products become.
So Microsoft would be more likely to lend sony a helping hand if necessary than not. In my opinion.