IMHO, the merging of Win 98/ME and Win NT/2000 took the life out of both. XP and every release since have been torn between the incompatible goals of pleasing both enterprises and consumers.
I think splitting Microsoft into two companies, one focused on consumers and the other on enterprises, makes a lot of sense. These two new companies should both get full access to the current Microsoft technology stack. And they can take it in different directions based on the needs of their respective target market.
We can again have a consumer Windows focusing on competing with Mac OS X/iOS and Android, leaving the boring enterprise market to the other company.