So... obviously nobody can predict the future. However, the reason the Nexus One was dropped off the upgrade list was likely due to some very specific issues with its storage and touch systems that massively limited its capabilities.
First off, it only had 512mb of internal storage. That's it. That, by itself, is highly limiting - and is part of the reason Google's shift towards one big block of internal storage makes a lot of sense. If, in the future, it makes sense to not have a /sdcard partition, or if more space is needed in the primary partition, you just resize the layout and keep on going. So there is a major limitation that's been dealt with - "future proofed" to some extent.
Second, it had a super crappy touchscreen. If I remember correctly, it was two touch points only, and was not all that accurate - it regularly had issues with switching of the axis using two fingers. With the shift towards on-screen buttons, and more multitouch gestures, it's no surprise that Google doesn't want to deal with that. That, too, has been dealt with as newer controllers like the GNex controllers support 10 touch points - all of your fingers. Thus, again, ostensibly "future proofed" since there's no reasonable way to actuate more than 10 touch points.
All of that adds up to me being pretty confident that the GNex will be supported for a long time. The Nexus S will be receiving ICS as well - I think that device will get a long support lifecycle, too.
But what apps are properly utilizing quad cores?
