Hal's first line of reasoning is based on basic engineering philosophy - if you have two things that do similar things and you want them to behave in a similar way, then you should unify them. Although, I'm not sure that Windows NT could be modified and branched to support a mobile environment with the all of the unique challenges that it has. Does Apple have a unified OS for its desktop and mobile platforms? No. Mac OS X and iOS are separate because the experiences will always have fundamental differences.
Berenson mentions many OS features that are not present in Windows CE that could be added with a lot less energy and resource committments from Microsof if they were to just replace it with Windows NTk. Some of the features he mentioned included full drive encryption, dual core support, and SMP.
In the blog he also draws parallels to the development of the .NET Compact Framework and explains why he thinks that Windows NTk can be small and modular like CE:
So let me address a key objection to the move, “size”. Windows CE has a reputation for being small and modular, Windows has a reputation for being big and bloated. But Windows itself is about 6 years into an effort to completely restructure and clean up the code base, its resource requirements have actually shrunk over the last two releases (Win7 and Win8), and Windows 8 is very much focused on the low power/constrained resources environment that characterize both tablet and phone devices. Indeed I am running the Windows 8 developer preview on a tablet that is less powerful and has no more memory than the current generation of smartphones and even at this early stage it is as responsive as those devices. In some regards, such as boot time, it is actually faster! So could a WinNTk-based Windows Phone 8 run well on a 1Ghz CPU with 512MB of RAM? Of course it could. What about 800Mhz? Almost certainly. 256MB? Well, now I start to wonder. -- Hal Berenson
Check out his full argument via the source link below.