If this were really true, why is there the existence of link rot and a large volume of online lost media?

I think the proper way to say this is that “if you post it on the internet, you should consider it being there forever”.

For example - a personal one. I did a short ambient music podcast series highlighting artists who release music via Creative Commons (a new thing at the time). It was only 5 episodes, and I have the first one archived. The other four are now completely lost to time, despite being put out on the internet back then. It’s not there forever.

In terms of social media, it’s harder to not be forever, but even that’s down to the same issues - has someone else archived it, screenshotted, especially in the case of a site ceasing to function? Internet Archive doesn’t preserve everything either. Plenty of archived pages missing images or files that enable true functionality to view everything as it was.

That maxim is no longer true, especially in the medium-to-long term. Reddit posts older than 8 or so years old aren’t accessible (even to their creators), same with the original Digg posts. If you go back to the Usenet days, most posts haven’t been archived and are lost, especially those from the smaller newsgroups. I expect over the coming decades, we’ll see data loss as a growing issue.