My guess is that RIAA will pursue it until they either win legally or exhaust and frighten everyone away from it and it becomes a much less powerful tool. It's not like DMCA notices are their only option after all (and I'm not sure the DMCA notice sent to GitHub is really even a DMCA notice anyway). I would imagine they're either already in the process of or are ramping up to also go after the individuals involved as maintainers and (major) contributors. I think that could actually be a really interesting legal case to see if something like youtube-dl is found to be something more like a VCR or more like Napster, but I don't know if RIAA would ever take the chance of letting that happen on the chance that they lose. I think the more typical strategy for them would be to start sending everyone they can track down the real identity of scary letters and maybe drag a few into court and settle so the rest run away.

I personally wouldn't touch contributing to youtube-dl right now with a 10 foot pole, but I'm also a bit on the paranoid side :).