I don't see this ever happening as plex is designed for broadcasting/streaming your own library, not sharing your media with other servers. From a legality standpoint Plex is already in a sort of grey area in regards to backing up your media library from the original disks. Adding a feature like that would be pretty implicit file sharing of copyrighted material.
If you wanted to do this outside of plex, might I recommend rsync or something similar? There are plenty of solutions for backing sstuff up.
Early 2021 clean-up: instead of replicating the media to another server you can share with your friend/family member’s Plex account, letting them stream from your server (considered implemented)