On the surface, that works. Problem is, to use the Fediverse you have to get a bit deeper into it than with email.

Email is designed to evoke the UX of the physical post office. To use the post office, or email, you need to know your address, and your recipients address. You need to know where to put outgoing letters, and where to get incoming letters. Even if you're vaguely aware of Grumman LLVs and letter sorting machines and trucks and trains and whatnot, you can still get away with conceptualizing it as, you put a letter in a box, it is then "In the mail" until it is delivered to the recipient. Email presents itself to the end user as exactly that.

ActivityPub might be "just another protocol" like smtp or pop3 or whatever but the user experience is vastly different in ways people really haven't had to deal with before. Lemmy isn't lke the post office, it's like Reddit, except there's 90 little Reddits each with their own slightly different rules and a complex web of which will communicate with what. The format of the electronic communique is of no consequence to the end user.

On Reddit, if I write a post in a subreddit and click Post, it is stored on Reddit's servers, and anyone with a Reddit account can access Reddit's servers and see it because we're accessing the same monolithic system. On Lemmy, I'm currently posting to lemmy.world from a sh.itjust.works account in response to an account from programming.dev. On which of those three independent platforms will this message be stored? How could someone from, say, piefed.social see it? I genuinely don't understand this fully msyself and I've been on Lemmy for a couple years now.