> what would stop some future societal development from inspiring the FSF to release a GPL v4 that says "this source code is exclusive property of Microsoft to use as it sees fit"?
Nothing, but the "or any later later version of the GPL" clauses have protection against this scenario. Section 14 of the GPLv3 contains the nice sentence "Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.", which is essentially legalese for "if it differs in the spirit of the license, it is not considered a new version of the GPL and the upgrade clause does not apply".