The problem is that this doesn't happen often enough. It's a very difficult thing to do, putting your baby up for adoption.
So what GitHub should do is a little bit of social engineering to encourage inactive maintainers to bless a fork. It's more than that, of course: you have to ask somebody if they're willing to step up. It's unlikely that one of the forks is comprehensive but there are likely people that would be willing to make their fork comprehensive rather than see the project abandoned.
Handing over the reigns to a committee does not work very well, in my experience. It's better than abandonment, but that's all you can say about it.