This is like a Tire company re-inventing the wheel for each new tire they make....
Why not work to optimize the current code and maybe reduce it’s memory footprint, maybe code a few things in assembly....
Microsoft will get to doing that as soon as it stops supporting an 11 year old, security-hole riddled OS named Windows XP that some Freepers claim to be the "best OS ever" and will never migrate from.
In all seriousness, Microsoft's problem for years has been that they attempt sometimes badly backwards compatability with legacy OS and programs. That inherently limits their ability to develop a more efficient OS kernel, "break with the past" and innovate - like Apple and Linux (I know, Apple = Linux.)
I suspect we've seen the beginning of change at Microsoft with Windows 8, which aside from its ugly Metro UI is a MUCH more efficient OS down to the kernel than its predecessor Windows 7 and is light-years ahead of XP in terms of speed, security, stability and resource efficiency.
Microsoft's also working towards faster OS release/upgrade cycles more in line with Apple/Linux so I'd hope that would drive innovation at Microsoft.
Time will tell.
(Disclaimer: I happen to use Microsoft, Apple and Linux OS' in my home. Even though I've been mostly a MS user since 1985, I much prefer Apple w/Ubuntu a very, very close second. But then I use each platform for different things....)