It would take a little more than Torvalds and Stallman changing their mind for Linux to become proprietary. Even if they did re-release their work under a common commercial license, there are many other authors that have contributed. If you look at what is included in the common distros, the list of contributing authors is rather significant. If it did ever happen, it would essentially be a giant code fork, and most likely others would continue development with the last free release.
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