Not for a very long time, 10 years I see is when the Desktop will basically be yes, a sandboxed environment for REAL legacy software. I bet that will be a feature for Windows nVeryfarNext Pro and/or Enterprise. If that were to happen, that will only be for Windows RT. There will be a turnover that will take place coding wise, Windows will need to support better app viewing, and developers will need to innovate UI design for touch and metro design at least to an extent. It seems Microsoft is working on getting the system programs that have been in Windows forever into a metro WinRT form with Blue going forward, so the Desktop will be used for third party programs. Office may or may not make it to a WinRT app within three to five years. Basically, once that happens, that will signal to the developers out in the world to start REALLY doing WinRT app development, and make it easier to develop their apps for Windows Phone. This has been seen as when Office 2007 came out with the Ribbon, a few years later, more notable programs started using the Ribbon design such as WinZip, AutoCAD, and some audio programs. There may be more that I'm not aware of. Although Office in the metro form might use the Radial menu design like in OneNote MX....
As for the metro core, or WinRT, I doubt that could happen soon. WinRT is built on top of the old Win32 layer, WinRT is basically a pretty finish with new plumbing that ties into the old, stable foundation. We'll have to see what happens, but since Microsoft is expanding on the MinWin kernel with Blue, this could happen quicker than we think as MinWin is pretty legit!
I agree with FuturDreamz on this as well.
Part of the reason to shift away from the Desktop code and UI is because that has been where Windows' issues occur. Poor app development that gets deep into the system files can easily corrupt things, and thus result in issues. For example, WinRT apps are restricted from the registry hives of Windows, so that prevents system errors (winrot) from ever happening. I think the self contained .xml files are how the apps are built.... Or malware, as I see and interpret it, it seems difficult for malware code to make it on a Windows OS with the WinRT apps. I would image it's possible, but MUCH more difficult than with the Desktop.