Cool URLs not changing isn't about people memorizing them or even really about personal bookmarks: it's about links.
Accumulating links to and from your content over the course of many years isn't just valuable because of SEO, it's valuable because you're helping to build a legacy of interlinked content. And that's a cool thing to contribute to!
If the link still works it didn't change in that sense. Redirects are totally fine and a way of implementing "Cool URLs don't change". Having redirects doesn't invalidate the principle.
Sure but hotlinking I don’t think makes sense anymore. Every link will eventually die so the only way to really ensure that links live as long as the content that they point to is to host a copy yourself.
There's multiple external links on this HN discussion page alone. HN would be a very different place if it insisted on mirroring all external content instead of using links
Accumulating links to and from your content over the course of many years isn't just valuable because of SEO, it's valuable because you're helping to build a legacy of interlinked content. And that's a cool thing to contribute to!