This made me realize that I don't actually love any website like that. Wikipedia comes the closest, but even if that would disappear, surely someone would have a dump, which I'd grab at that point. Until then, I won't do this.
The two things I go back to are music, and books. Even books not so much, but songs tickle a very specific part of my brain, that I want to reproduce over and over again, and then not for years, and then again. And when I want to go back and I can't, and I know I could, that really hurts, and I feel betrayed. So I do actually archive the music, even from streaming sites.