Or alternatively, github could rename this org since the owner is no longer in contact and then let the currently active group have the name.

Nobody is giving away the current org - github would just be vacating the old name so somebody else can have it.

This is bound to become a bigger issue in the future as more and more names get used then fall into disuse.

IANAL, but apart from a private entity having such power being problematic (to say the least) who's going to put stuff on GitHub if it can be gamed to take away your control?

Edit: GH could also be in for liability claims should the original owner re-appear (at least in general, if not this particular case), and would need to check identity of the involved parties and whatnot, which is going to be very costly.

What's with all the legal bluster in your comments here? Not only do the liabilities you're trying to conjure up not exist, but this is not even the first occurrence of something like this happening with GitHub. They have a documented policy for freeing up inactive names. (Spoiler alert: it's allowed, and they've done it.)