Partially true.

If StackOverflow dies because nobody uses it anymore, the entirety of its knowledge is now only available through LLMs that trained on it.

If a blog dies out because the author gave up writing, the entirety of its knowledge is now only available through LLMs that trained on it.

If people stop writing books because they cant outcompete generated content, that knowledge is also lost to LLMs.

If people stop making art because they can't outcompete generated art, that is also lost to LLMs.

So yeah, they haven't directly taken anything away, but the consequences of what they are doing may still have that outcomes - and it does look like that's the version of reality we're about to get.

> If $X dies because ..., the entirety of its knowledge is now only availabe through LLMs that trained on it.

And archive.org. And scraped copies people have around for various reasons. And libraries. And even in the first-party source, should they just leave it be instead of shutting down and destroying copies in pure spite.

The knowledge did not disappear, and it shows no sign of disappearing faster than it loses value - which is the usual case, as all the examples you gave always come with an expiry date. For StackOverflow, that's measured in low years; for blogs, high years to a decade. Past that point, we enter the realm of curating and preserving knowledge past its commercial utility expiry date, which is a separate endeavor, and one that LLMs not only don't threaten, but actively aid.

You are mixing up culture, data, and the creation of new works as if they are the same thing when they are very different.