The key to adoption for anything like decentralized and secure communication is simplicity and ubiquity.
We added secure messaging to Outlook in the late 90s. It could be enabled by default. It was pretty easy to set up. The problem was that anyone who used something other than Outlook (and the latest version, only on Windows) would have a really hard time enabling security. We had passed the simplicity test but failed the ubiquity test.
What happened of course was we all sent/received secure email. And then one person couldn’t read the email because he was on a different client for some reason. So we would send another copy in plain text. Which defeats the whole purpose. And soon, no one used secure email and we were reduced to least common denominator.
So something like BitMessage could be wonderful. But it needs to have a fallback and accept that for many recipients, especially at first, there needs to be a backup. Somewhat like how GroupMe or Glympse will fall back to SMS BitMessage (or another secure, decentralized system) needs to understand that falling back to email isn’t necessarily a bad thing, just a temporary step to getting more adoption