It doesn't need to completely replace the current platforms. The beauty of the decentralized internet is that platforms suddenly disappearing/dying wouldn't mean we lose years or decades of information that was contained in that site/forum/corporate entity.
Decentralization would also encourage a lot of people to go back to blogging, which would mean information would come from all over the web again.
Mastodon doesn't list old post if it'd from another instance. Maybe it's a setting, but if a server died and the content isn't backed up, it will disappear. But the community can move to another instance.