Good question. There were some notorious cases where trolling or pileons made it advisable to shut down a thread altogether, and folks lobbied GH for locking as a solution for that. Definitely mark threads as "Answered". For now, let's reserve locking for situations where rekindling a conversation would be inappropriate for whatever reason. We can always use it more freely later if we want. But a good starting default is openness to the extent good community behavior facilitates it. If someone wants to resume conversation on a stagnant Discussion, it likely isn't more harmful than resuming conversation on a stagnant Issue. And GH gives us tools to move it if need be.