Etherpad was completely unmaintainable when it was shut down as a service and open-sourced. Its first, open-source major rewrite (etherpad-lite) didn't work. It took a second major rewrite by a profit-motivated startup (Hackpad) to become useful again.
I think that if Etherpad had been GPLed, we would still have nothing but etherpad-lite, and we would lament Etherpad as a lost technology.
