In talking with some of the editors of AnandTech, apparently the limiting factor here is how DirectX applications would react if you were to disconnect the external GPU from the device and Microsoft would need to be the one to architect a better system reaction than a blue screen of death. There is one MSI demo showing this working, but it was never released.
So maybe 1-2 years? Heck, we'd need Thunderbolt on more non-Mac computers first.
iMac with Retina 5K Display | 15" MacBook Pro with Retina Display| Mac mini | iPad Air 2 | iPhone 6 Plus