Microsoft is clearly on a path to remove WMC from future Windows operating systems, so I am planning to keep my Windows 7 HTPC (and Xbox extenders) around for another 3-4 years - which will be about the time I predict HD video streaming will actually be usable, reliable, and will likely have content people (like me) want to watch. WMC has been at the center of my entertainment system since Windows XP Media Center Edition and now to the current HTPC running Windows 7 with 2 analog NTSC tuners, 2 digital OTA tuners and 4 digital cablecard tuners (via Ceton). However, I never considered WMC really "usable" as a general consumer HTPC until Windows 7 when anyone could add a digital cablecard tuner into their HTPC - this is the point where WMC could truly be a replacement to a cable company box or a Tivo. So really, history should consider the "success" or "failure" of WMC starting with Windows 7.
I expect the future of "WMC" to be with Xbox and third parties like Ceton. We'll have to see where Microsoft goes with Xbox and whether we can ever easily add a Ceton digital cablecard tuner and a couple TB of storage to record digital TV and extend those recordings to other Xbox devices.