Here's a time when I am grateful that ello allows >140 characters.
The key line in question is: "However, with all their wizardry, perhaps Apple and Google could invent a kind of secure golden key they would retain and use only when a court has approved a search warrant."
Apart from the amusing wording, I'm actually generally ok with this, and I suspect Snowden would be too. The key difference between what the NSA has been doing (the dragnet surveillance) and what WaPo seems to be suggesting is that this is targeted. You must have a warrant, thus there is a fact pattern, attribution, and in theory accountability.
Maybe the issue is that it's too easy to get a warrant? Or maybe people feel that judges will randomly issue warrants, and when it is abused there will be nobody held accountable? I'm not sure. But if someone has probable cause that a person is using an encrypted cell phone to do something highly illegal, like engage in a terrorist plot, and things like the date and time of the call/text can be articulated, then this is actually really useful.
That said, what works legally does not work cryptographically, and I'm really unsure whether this could be implemented in a way that does not guarantee against abuse.