When they say merge, if one has listened closely, is that they will all be built from the same Windows NT core. Which means almost all prior versions 98, XP, 2000, Vista, 7 and 8 were built using variants of the NT core. But now, there will still be different versions but they are to have the same NT core. At least that is my understanding.
So really the new core, I suspect, will have variations from of all the past models. Gee, my explanation almost sounds like double-speak.