There is something like that called Plebbit, it's a good example of all the downsides of such a system.

It's decentralized and "censorship proof" AKA completely unmoderated with all the spamming and unsavory content one might expect. The CSAM issue is mostly avoided because it doesn't support any kind of media sharing (only plain text) due to the bandwidth for P2P connections and avoiding having to store that on the client. Last time I checked content also simply disappears when users disconnect.

The federated system is better because it's more efficient and scales better since only a couple hundred instances have to send/receive activity instead of hundred thousands of users individually. It also helps ensure content is moderated. Admins tend to keep their own instance clean and defederate from instances that don't.