It's not just the lack of backups. It's a world of constant pain and worry about your only device, security keys, and safe storage and transfer of your data. Forgot password? Get new account. Lost a key? Get new account. Want to open your messages in a browser from a desktop computer? Welcome to a world of pain. Your device switched off? Then nobody can send you messages, because in 'true p2p' there are no relay servers who would store the data, waiting for you to connect.

In practice, nobody wants any of that. The vast majority of users are perfectly happy (and for the right reasons) with federated and centralised services like email or WhatsApp/etc.