> but we decide what "open source" means
Well, "we" in the Open Source movement decided that it means not restricting what users do with their code.
I don't understand what's so hard about this. Overwhelmingly, the biggest players in the Open Source ecosystem have consistently been saying, "call it whatever you want, just be clear that you're not the same as us." And the response from a lot of people has been, "no, you have to let us call ourselves the same as you."
Source Available has been a term for a really long time, and its generally understood what it means, and plenty of people use the term with plenty of good will. This shouldn't be a controversy, it's such a straightforward, easy concession. There's nothing wrong with being Source Available.
So why do we need to come up with a new term that describes the freedoms we've been consistently preaching for decades just because a couple of for-profit companies want the good will that we've generated? This isn't language evolving over time, it's people outside the community trying to decide how we're allowed to define ourselves within in the community.
Yes, language is descriptive, but that doesn't mean you can just redefine anything you want any time you want. Even more than that, the fact that language is descriptive does not mean it's a bad thing for communities to try and publicize and preserve the descriptive language that already exists. Open Source advocates are hardliners on this issue specifically because we recognize that language is descriptive, and that our ability to preserve the meaning of the words that describe our community is dependent on us taking the time to educate and correct people who are genericizing those terms.