These mergers will not happen and I already questioned you in the other thread about the ridiculous assumption that MS won't even be around for the next console generation. The Xbox360 is already way more successful than the Sega Saturn, and they still stuck around to release the Dreamcast. MS isn't going anywhere, nor do I think they'll merge with Sony for any reason.
I'll predict it here, in fact: MS won't even use Blu-Ray disks as their software medium for the next gen. What's more likely is they'll use a modified form of HD-DVD or another format altogether. Why do I think this? I don't think even Sony will be using Blu-Ray next time. Nor will Nintendo who seem to be trying to develop holographic storage mediums for the next iteration.
If anything, the next console generation will include a new entrant: Whatever the hell Apple is cookin' up, or, if things get really perverse--a console built by EA. But next time, I feel pretty confident there will be Nintendo, Sony, and MS all building machines and very likely, a fourth competitor will come into the ring.