Not being able to move elsewhere is a lock-in issue (Matrix has an issue for decentralised identities [1]).
I don't see why we couldn't just attach our identities to a private key. Encrypt that private key and back it up on various online providers (using PAKE so that they never see it). Use those to authenticate elsewhere. Designate one, or multiple, providers as a temporary mailbox (rather: ask them to fetch content that you are a recipient of from a DHT). People would identify you with pet names they assign to you. You could publish your key in multiple places (in my HN profile, for instance).
I think of a DHT rather than a blockchain, because blockchains don't scale. Your user account could itself be a blockchain, though, stored encrypted in the DHT.
Not sure why I haven't found such a system yet? Secure scuttlebut seems to come close, but I mostly want an authentication and message passing system, that e-mail could be re-built upon, and bridged with.
The major problems I see with slack and similar is that it is too easy to lose messages. If I click a channel, but get distracted before I can properly read or act on it, the message is effectively gone forever (unless I remember, or the next time I happen to open the channel). No other medium has the explicit "delete" (or archive or whatever) that email has. I find this way way way more useful than the "read" concept. Because read means nothing to me, even if I actually did read it (not that it was just present on my screen for a couple of seconds) it is incredibly likely that I forget about it.
Other things I like about email: - Multiple clients let me use email how it works for me. I can control how it notifies me, how it sorts my messages and everything else I care about. Slack is ok, but I don't really believe that one size fits all. And I'm not sure that it even fits many people. - I can send an email to anyone. I don't need to invite them to my workspace or channel or anything. Just enter an address and it is off.
If you push Command-LeftArrow (on Mac) it's like a back button for Slack channels.
A chat-view in email clients could counter the development, if messages from contacts have visual advantage over automated email. The Ivelope client was featured here before, but went quiet. DeltaChat tries to do it, Chat-over-IMAP is another campaign of vendors.
Problems like mailbox size, search and indexed view of attachments/content, expectations on privacy.. this friction will remain from a user perspective.