Do all of the implementations, particularly on mobile, actually provide such security? If a court is willing to order TLS keys to be turned over in order to enable the government to access information, why couldn't they order the developer to release a backdoored version through the relevant App Store or other update mechanism? If the author refuses to comply, then they will demand the codesigning keys so they can backdoor it themselves.
To quote your critique of another's security proposal: "Then the only difference between hushmail and your model is exactly what the FBI will get a subpoena to have you type into your server to subvert your users. The models are equivalently insecure."