But... there is a definitively written definition for those words. The phrase was invented to refer to a very specific thing. Changing the meaning of the word would accomplish nothing except force existing usages of the word to change.

Like, if fair source licenses began to be referred to as "open source", then "open source" will have lost its original meaning. So now when stating that something is "open source", you will have to clarify whether you mean "original open source" or "expanded open source" (or something like that). This distinction will be very important to potential users, since it may or may not restrict their intended use case.

It's no different from if we were to start referring to reptiles as mammals. Now when a biologist wants to refer to only organisms with mammary glands, they will need to use some other term, like "milk-making mammals". It does nothing but cause confusion.

Not sure how else to explain this concept... like, I'm really just talking about semantics and pragmatism here. I don't disagree with you on ideological grounds, if that's what you're assuming.