If EtherPad had been GPL I have my doubts Hackpad would have ever been forked from it or if Hackpad would have even been a thing.
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.
(Note that I'm not debating the GPL point. I don't personally think Hackpad's forking the code has had any meaningful effect on etherpad-lite's popularity; there are plenty of collaborative document editors out there.)