I'm not sure you're wrong. I was under the impression that contributors retain copyright over their portions under GPL.
This would require the copyright holder of the larger piece to get permission from all contributors before releasing under a different license, and I thought Ubuntu's CLA with Mir was to ensure they weren't trapped by this.
Nearly all companies who release stuff under the GPL require copyright assignation on contribution, actually giving the company far more control than if they simply used a liberal license. Using the GPL is more about making sure third parties can't lock up derivatives than that _you_ can't.
This would require the copyright holder of the larger piece to get permission from all contributors before releasing under a different license, and I thought Ubuntu's CLA with Mir was to ensure they weren't trapped by this.