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.

I too share your general optimism about the current state of open source. I really hope that you are correct in your conclusion that the GPL mandate is unnecessary because, as we see here, many projects are trying to get away from it. However, I feel we have a number of historical examples of what happens when constraints are lifted in a space that has a number of very powerful players. None of them look like anything I want the Open Source movement to look like.

Well, whatever the license, here is to a bright future! That a rising tide may lift all boats.

The GPL is supposed to put constraints on you, but I'm still waiting for a source code release from Billion for their router firmware (Busybox) and Philips have removed access to their GPL source trees.

Why do you feel that? There's no benefit to a company in maintaining an ever evolving and complex fork of a project as compared to contributing back the non business essential parts so that their fork can be smaller.