So would there be a use-case for a combined bot+app that you can connect some storage to (S3, FTP) which archives all channel message history in a searchable form? It could make it visible to search engines, searchable by humans, linkable to people outside of slack, and shouldn't take up very much storage space.
Should be quite trivial to build also, and cheap enough to run as a bit of charity if projects pay for their own storage.