There is no transition in the investigative state machine for "data relevant to our investigation to which we are lawfully entitled exists and is available to us, BUT we will not retrieve it because doing so would be invasive to other members of the service". It's possible --- we could do some research --- that that transition exists in the state machines of no western government at all. Knowing that: if it's feasible to obtain information from a secure message service, eventually, the courts will mandate its retrieval.
As of 2014, there is a fundamental tradeoff that we know for a fact exists: you can design an encrypted mail service that is trivial for users to adopt, or you can design an encrypted mail service that meaningfully resists judicial power. You can't do both.