> 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.