Windows 8 RTM is not ready yet, but lots of things have been updated already with regard to the new version of the OS. Among them the download page for Debugging Tools for Windows.
Here a screenshot from the page.
Notice the following paragraph:
There are certain rare circumstances in which you might need to download the Windows 7 version of Debugging Tools for Windows.
So working on Windows 7 and wishing to install the Windows 7 SDK is nowadays a “certain rare circumstance”. You wish, Microsoft!