Don't let perfect be the enemy of good.
You realize that by your own definition even the NSA is mostly a "weak passive attacker" They do *not* have the ability to attack more than a small, targeted, subset of connection for both technical and political reasons. For starters, MITM attacks are easily detected - "Bitcoin network attacked by unknown agents! Has your ISP been compromised?" would make for great headlines and would soon see the problem fixed both technically and politically.
In any case, my suggestion of enabling hidden service support by default adds both encryption and reasonably good authentication.