The current value of Twitter is not in it’s technology, it’s in it’s userbase. You could write a better engine to do what they do, but you can’t convince enough people to use it.
Your suggestion to decentralize essentially has them giving credibility to external sites (who licence their engine) at a cost of what they value most (their users). Once I’m buying t-server from them, a new startup can compete on features..<p>I’m sure there are lots of good ways for Twitter to make money, but I don’t think that diluting their user base is one of them..
The Exchange analogy doesn't apply to Twitter.
For starters, most people want to keep their Exchange email confidential. IOW, the rest of the world can neither see the contents of the "inbox" nor the "sent mail" folders.
OTH most people want their twitter status updates to be seen by others and there isn't much incentive to host it securely.