Well, I don't disagree with your specific points, but you are missing that the founding idea of FOSS was a copyright hack to ensure that software remains modifyable/fixable by its users. A mandatory CLA that allows relicensing partially circumvents that not only for the users but even the contributors.
Sure you can argue that technically the already released code can not be relisensed, but that's really missing the point that FOSS software intends to be open now and in the future.