Maybe you have a great concept, but as a general principle that sounds bad. If someone doesn't have your special client, it will be disjointed. If someone does have your special client, why didn't you just use a saner protocol.
Also, email is not really very decentralized. You need to be one of the major providers to avoid the spam filters.
I realized I didn't address the more important of your two points, figured I'd do that now.
> why didn't you just use a saner protocol.
Mostly because your email network with your friends and colleagues already exists. Even more - we already rely on email as the ultimate identity authority for just about everything based on the web. My idea simply adds crypto-based authentication, UI improvements, and chat over the top of that.
Emails are so simple and ubiquitous that people have already implemented email <-> help-desk bridges, this idea just takes that bridge concept and applies it to interactions between user.
Take a moment and picture what it would be like to have every thread in Facebook delivered to your inbox. It would be a mess with the current mailbox UI, but there's not much impedance mismatch. Now picture the reverse - every email you get shows up as a "post" on Facebook. Both consist of the asynchronous distribution of messages from one user to many others, it's just that one relies on Facebook's network of servers, and the other relies on mail servers.
It would not be any more disjointed than any other form of email exchange - just not as well organized as a social network.
> You need to be one of the major providers to avoid the spam filters.
Yes, but you could take advantage of the existing email providers' IMAP and POP implementations, and companies have long worked to ensure their emails are not sent to spam folders; it's not entirely a black art or impossible.
Also, email is not really very decentralized. You need to be one of the major providers to avoid the spam filters.