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.
Local client logging doesn't solve all problems (though yes, i still want to be able to grep text files if I choose).
The two I regularly encounter (which shared storage fixes) is either moving to a different device (or trying to remember which ones have which bits of history on) and also integrating new team members.
The latter is especially important if you're doing anything technical/support related; if you've had 2 days of discussions and want to add someone else in, they need the history, not for you to try and cherry pick the important bits of the last few hours to then fail to get them upto speed.
Should be quite trivial to build also, and cheap enough to run as a bit of charity if projects pay for their own storage.