Nintendo and MS could both easily buy sega, and they both aren't strict on their developers....But it doesn't look like sega wants to be bought. They want money, and the only feasable way is through a merger. Sure, if they are bought, they will be provided funding, but who wants to be bought? But the problem is Nintendo and Microsoft are just way to big and rich to want to merge with a puny company like Sega. What would Nintendo or MS benefit from a merger? Exclusive games, sure, but they would get that in a buyout too, and in a buyout Sega would not be their "equal", they would be 0wNeD!!
I don't see why Namco would want to merge with Sega, unless they stated a lot of conditions making Sega the less powerful side of the parternship, which they probably did.
And if Sega did finally go for sale, out of the 4 companies that can actually afford to buy them out without making dent in their finances (EA, MS, Nintendo, Sony), Only EA and MS seem like they will bother buying them. Sony doesn't have a history of really buying too many developers, especially ones that have asking prices in the billions like Sega probably does. Nintendo on the other hand has bought (both completely and enough to make a second party) plenty of developers, but almost all of the time they have been companies with a long history with Nintendo (like HAL) or companies that are just starting or ready to go into the big leagues that Nintendo develops an interest in (Rare, Retro, etc.). The only real exception is Silicon Knights, but they were pretty small, unknown, and cheap (compared to Sega). As for EA, it depends on if they want to spend billions of dollars on Sega or not. They seem more content with publishing games from their own studios or independent designers like Black Box than buying other companies...MS on the other hand, like Nintendo, has bought or second-partied several companies, and they ain't afraid to spend money if it will help their console (unlike like Nintendo, who says it's not in their "business philosophy" to buy other companies, which is sorta hypocritical...).
So if they do go for sale, I can see MS jumping up and down at the beginning of the line...Sony doesn't seem like it will even care too much (but you never know), and EA will probably just watch also, but then again they HAVE shown an interest in Sega. Nintendo has billions and billions hoarded up that will probably never even be spent, and I think it would be foolish if they passed off the oppertunity to aquire sega. Why not spend a billion or two and secure yourself tons of exlusive games, especially when those billions would have just been sitting in some Japanese bank for all eternity? Then again, in the long run they would probably have to spend a lot of money maintaining Sega (Rare comes to mind), but hey, Sega would hopefully bring in profits too, not just spend money...But I don't see Nitnendo changing heart and buying sega...