My impression is that, yes, Durandal will be dropping Knockout. Not because of any issues with Knockout, but because they (he) are aiming the NextGen version at new, browser native technologies, such as Object.observe, WebComponents etc. It's all very bleeding edge, but I can't blame them (him) for wanting to stay ahead. I believe the Knockout version will continue to be developed, for quite some time. I also think it's likely that Knockout will eventually also build upon upcoming native stuff, with a few fallbacks for older browsers.
/G