Unlike the Great Library at Alexandria, the information contained in many reddit threads is actually available in other places and can be recreated - often by the same person if necessary and relevant.
I understand people not wanting to have that information deleted, but I think the analogy is a bit heavy. For many, it's a balancing act where the fundamental disagreement with reddit's cultural evolution outweighs the desire to participate in the knowledge repository.
I think many people were comfortable with their ideas belonging to the communities that spawned on reddit, and they viewed reddit's ownership as a necessary technicality for the platform to exist. Once reddit clarified that they intended to act on that ownership, many people no longer wanted to participate.
I think they have that right.
More importantly, who owns our thoughts in this space?