I'm not sure this is true (anymore?). There are many open source projects who are thriving under non copyleft licenses.

If someone wanted to be a dick about it they could release modified GPL code as one big diff and publish it on some obscure webpage only their customers have access to. Instead we see more and more companies and individuals becoming friendly open source citizens. Perhaps some because the GPL mandates it but I would like to think mainly because they feel it's right thing to do—copyleft or not.