The difference is, email isn't (usually) public. A status message is there, for anyone to see it, like a tweet.
Re:Dave Winer: An open Twitter-like ecosystem
by look_lookatme on Friday July 27, 2012 @04:15PM
(#4303377)
If a message is posted in public and no one sees it, does it matter?
People won't be using this new decentralized service. They'll be using Twitter. I guess you can feed into Twitter, but the last mile is still Twitter.
If you have the infrastructure to handle a setup like that, you should already have an internal email server.
EDIT: After your edit, I think I've found where the disconnect is from my perspective. The problem with this method is that discovery(arguably the most important part of what Twitter provides) is still reliant on a third party's index.
From a user's perspective, Twitter provides 3 key services from one URL:
1. A unified feed for everyone you follow(This proposal also does this).
2. An easy way to post/host content(This proposal does not deal with this).
3. An easy way to discover new people to follow(This proposal also does not deal with this).
Out of those three, I would argue that the second and third are the most important. The problem isn't getting the message out to people that are already subscribed with Twitter, email, or a hosted website. The problem is discovery, and giving people an easy way to actually find the information that they're looking for.
The only way to handle discovery on this way is to have some hosted, third party method of searching through the users to find the ones you want to follow.