Whether or not its "invalid" isn't the point. Those are the accepted terms by most people, especially those in the industry. The point of language is to communicate ideas.

When most people say "free software", they're talking about software that's free as in freedom. Using it otherwise just causes unnecessary confusion.

There is no path to any future where someone will be wrong to use the word "free" to describe software that doesn't cost anything.

Setting aside that doing so is already misleading, you clearly lack imagination if you cannot think of any feasible way for that to happen.

For example, consider a future where use of the phrase when advertising your product could result in legal issues. That isn’t too far-fetched.

They don't become invalidated. They're not capable of becoming invalidated.

They certainly can. A given meaning of a word is invalidated if it is no longer acceptable to use it in a given context for that meaning. In a medical context, for example, words become obsolete and unacceptable to use.

Likewise, it isn’t valid to say that your Aunt Edna is “hysterical” because she has epilepsy.

But more importantly, that’s all beside the point. Words don’t just have meaning in isolation - context matters. Phrases can have meanings that are different than just the sum of their parts, and saying a phrase but meaning something different won’t communicate what you meant. If you say something that doesn’t communicate what you meant, then obviously, what you said is incorrect.

“Free software” has an established meaning (try Googling it or looking it up on Wikipedia), and if you use it to mean something different, people will likely misunderstand you and/or correct you. They’re not wrong in this situation - you are.

That, or you’re trying to live life like a character from Airplane!:

This woman has to be gotten to a hospital.

A hospital? What is it?

It's a big building with patients, but that's not important right now.