Just to be preemptive, advocates on any side of any position can have a vested interest in a particular outcome. We should always be skeptical.
On the second point about refactoring/re-engineering you've overlooked some facts. One is that Apple has releases like Snow Leopard that are primarily a refactoring of the previous version in order to set the stage for future improvements. They also replace frameworks (Cocoa versus Carbon) instead of layering one on the other (Windows API versus MFC). Microsoft, on the other hand just keeps piling it on and is, by Gates own words, more interested in features than fixing underlying issues. Linux and *BSD also do a good job of keeping things from gunking up by avoiding a lot of tight coupling of features to the OS kernel and each other.
So yes, Windows NT was not only new, it was MS's first full operating system. Windows was until then a GUI environment on a control program/monitor - not a full blown OS. However, NT is big and has gotten bigger thanks to Microsoft continuously adding on new tightly coupled layers, e.g. COM/ActiveX, Windows MFC and above, etc. Remember how MS argued to the EU that they couldn't remove IE from Windows because it was so tightly coupled to the OS? Relative to Apple and the *ix communities, Microsoft has not managed the underlying OS architecture very well and that makes it difficult for it to make non-superficial changes quickly.
Microsoft has a business model that demands it make big bucks selling the OS because it doesn't own the hardware business. They've also been incented to tightly couple applications and features to the OS in order to preserve their market advantages. That has led it to where it is now which is an OS that doesn't manage power use very well and is difficult to change. The bottom line is that Windows performance in terms of power management is due to Microsoft's decisions and is not due to the fact that its a desktop OS.