The longer they wait, the less hope I have for a Greatest Video Hits III.
However, with the new 2011 anniversary push, there's the possibility of either a new collection of videos or a re-issue and completion of the current GVH series.
If it makes you feel any better, other bands have been in similar situations, such as AC/DC, who released their 2 disc "Family Jewels" video collection, which stopped short of their last few albums. I believe a limited edition of the "Iron Man 2" soundtrack attempted to tidy up the remaining videos on an additional DVD, but it certainly wasn't a proper, stand-alone release.
Rush has come close to covering all their videos, between "Chronicles," "R30" and "Retrospective 3" on DVD, but still fall short of a few classic tracks which appeared on the now-long-out-of-print "Through The Camera Eye" VHS.
So, yes, for as many bands and artists who have accounted for all their official videos, just as many have a haphazard scattering coupled with missing ones. It seems that few put much effort into the gathering up of these gems.