Clients can download reasonably large archives reasonably well. You kinda want to avoid having lots of connections per comment. Even the most recent content can be combined into an archive (along with public keys) and passed around between nodes (favoring nodes that forward it faster and/or have the least invalid signatures) If someone puts a large video in their post it can be a separate torrent that automatically survives depending on the posters popularity and the quality of the video. Those who desire it can pretend it is a block chain and hoard everything.