A party simply can't change the license for code that they did not write. They can only change the license for their own work.
The FSF avoids this problem by requiring copyright assignment. To be honest, I'm not sure that's enough, but I think they're hoping for a sympathetic judge. Most projects have never looked at what the law actually says regarding joint work, and think it works the way they would write the law. I know what the outcome of that will be.
It's like claiming that someone shouting outside of a recording studio has the right to relicense the music created inside the recording studio. While it might be true, I'd want to see an actual legal case before claiming things that are that outlandish.
Not to mention that the GPL has been tested in court and has held. So there's that.
I know what the arguments would be, but I'm not sure how the court would rule. The hypothetical depends on legally making a fork, and the GPL (as an example) only gives permission to fork if you agree that any distribution will be under the terms of the GPL. If you violate those terms, you've committed copyright infringement. But can a joint copyright owner be guilty of infringing his own work?
I remember Eric Raymond discussing a commercial copyright infringement case where a contractor was hired to update a program, and started distributing the updated program without permission. This was his argument.
However, he hadn't made significant changes to the program. Using your words, the program was several orders of magnitude larger than what the contractor added. The court got creative, and declared the contractor only had a collective copyright -- like somebody who has copyright on a single article in an encyclopedia -- and not a joint copyright -- even though his changes were meant to be combined with the original work to create a single work and not a collection of works. A collective copyright owner only has copyright in his contribution.
You are right that courts would be reluctant to allow the tail to wag the dog, but there is some threshold where the contributor has made enough changes that he has joint copyright, and at that point he can do what the law says a joint copyright holder can do.
Every individual contributor could also potentially sue Microsoft in that case. The FSF wouldn't even have to lift a finger, and the contributors could get a nice class-action lawsuit going.
So far, open source projects act like copyright either requires unanimous agreement among the copyright owners to make changes, or each contributor has copyright only in their specific contribution (so, in practice, you need unanimous agreement). That's simply wrong for joint works, and joint works are defined as works meant to be combined together to make one work (the alternative being a collective work, where the parts are simply combined into a collection, does follow the "each contributor has copyright in just their contribution" model).
Copyright law is based on certain assumptions, like "if the co-authors get into an argument, it will be about how to divvy up royalties, and there will be a contract involved." Another is "co-authors work together at the same time, and know each other." Those assumptions are wrong for open source projects.