In theory, you could layer an email UI over a federated secure messaging protocol, like Matrix's E2E or XMPP+OMEMO. In practice, no one has done this, and infosec people only seem to care about instant messaging for some reason.
Besides all of the points you make above, Signal Desktop is terrible. I often want to use it to avoid typing on a phone keyboard, only to find that the experience of using Signal Desktop is actually worse than that of typing on my phone.
In theory, you could layer an email UI over a federated secure messaging protocol, like Matrix's E2E or XMPP+OMEMO. In practice, no one has done this, and infosec people only seem to care about instant messaging for some reason.
Besides all of the points you make above, Signal Desktop is terrible. I often want to use it to avoid typing on a phone keyboard, only to find that the experience of using Signal Desktop is actually worse than that of typing on my phone.