Most of the time when rides break down, CM's don't have any idea when it will come back up because of the scenario you describe above. On rare occasions when the ride closure is not due to mechanical failures, they may know. In both cases, the message given is exactly the same. I don't assign blame or even get angry about it, because I understand why it's done, but I am acknowledging that it happens from time to time.
I go rather frequently so ride closures barely affect me. I can't even begin to imagine the frustration and disappointment of others that don't get to go very often. I feel bad for people like the OP. These things happen, but I'm not sure that lessens the disappointment much.