Yeah - there's a ton of immutable URLs on the web. All of CDNJS/JSDelivr/Google Hosted Libraries, most of raw.github.com, all Imgur & Instagram images, all YouTube video streams (excluding annotations & subtitles), and all torrent file caching sites (like itorrents.org). There are probably some large ones I'm forgetting about, but just mapping immutable URLs to IPFS could probably cover 1/3rd of Internet traffic.
Check out https://github.com/ipfs/archives to learn more.