Email's a unique protocol because it's structured fundamentally differently than every other messaging protocol/app. It's focused on sending documents around, which are individual and have no external context. This is unlike most messaging apps which are chat focused: the base unit of interaction is the chat thread/channel/DM/whatever, not the individual chat message. Email's not really good for chat. You can have replies and stuff, and you can have threads, but because each email is independent and threads are made up of ad hoc links between messages, if one message forgets to include threading or reply metadata then the thread is broken. Modern messaging apps have a first-class concept of a thread or channel that messages belong to, which makes breaking conversations much more difficult. At the same time though, not everything needs to be a conversation. If I get a weekly newsletter, I think of it more as an individual document than a message in a conversation. If Amazon tells me my order is shipped, it doesn't really have any relation with the other times they've told me an order has shipped, so it doesn't really belong in the same thread of conversation
So many articles I read about "email needs to go" don't understand that messaging apps/protocols are conversation-based, while email is document-based, and that the latter model does have its benefits and should be retained. Yes, whatever replaces email needs to be federated and open instead of closed off like Discord or Twitter or whatever, but that's in addition to the fact that it needs to be document-based as well