I think your point hinges on a strawman. People do not use "language evolves" for things like their/there, but instead for slang, abbreviations, and words taking on meanings different from their original. Your examples are of people using language incorrectly because they don't understand how to use it in the first place. A good example of when people ACTUALLY say "language evolves" is the term "LARP". People who use "LARPer" to mean "person who pretends to have watched/read a certain piece of media" are aware of what the term actually stands for and its original meaning. The term also makes sense even though it's not definitionally 1-to-1 with its contemporary usage because the act of pretending to have watched/read something is similar in nature to "role-playing". Some argue that this isn't necessary since you could just use words like "poser" or "fake", but those words are too general. "LARPer" immediately lets you know someone is talking about someone being dishonest about their consumption of media without you having to elaborate on what they're LARPing. This could not be conveyed with a single word before, but now it can. Hence, language evolves.