GCC itself has definitely been inhibited by the copyright assignment. There have been a few different ports that were not upstreamed specifically because of the CLA. AVR32 and C6000 come to mind off the top of my head.
Can you think of a single instance in which code written by an individual contributor not bound by an employment contract was prevented from being incorporated into a project that required copyright be assigned to a central entity?
I have read multiple accounts of open-source contributors who have successfully negotiated when being hired that copyright to any code written on their own time on the own hardware will remain with them (so as not to complicate their contributions to open-source projects).
1: TBH it's hard to tease apart the cultural aspects from the legal. Most GNU projects were roughly "We'll post a tarball once in a while and not have public CVS" when I first started getting involved in OSS; projects with public CVS it was a lot easier to tell if someone was already was working on something, particularly for people as shy as I was back then; I got the impression that the "proper" thing to do would be to introduce yourself on the mailing list, propose the idea, &c. and then start working on a patch.
My particular combination of low-confidence and misanthropy steered me away from such projects.
(It doesn't help that the FSF's processes for copyright assignment are incredibly clunky. It was a big deal when they started accepting scanned papers in 2012, for example.)
Basically I trust the FSF to stand by the spirit of open source. I don't trust that out of Google.
And that's with trust that the FSFs goals on copyright align with mine.
I don't trust Google to do what I want with my assigned copyright.