>In an alternate universe, messaging on the Internet could have been:

I don't think so, and I think the very reason is because the people who opt for these decentralized solutions never really sit down and try and design a product, they just want decentralization for the sake of decentralization. For them decentralization is the product. Your alternate universe evaporates when you ask the question "what happens if Alice's device is offline?".

If you squint, the exact system you are describing is e-mail, and that has become effectively centralized, and it happened long before we had tech mega corps.