GreenGranite's point is well-taken. It's the paying passenger who puts the bread on my table and I'm grateful for it.
PAs explaining delays should be made and they should be honest and informative.
That being said, in this instance if the captain felt that passengers were so angry that he questioned whether they would follow flight/cabin crew instructions in flight, or possibly even initiate a physical altercation, he was doing the right thing in pulling the plug. I'd like to hear his story and reason for taking the action he did.
In 32 years of flying I've never seen a flight delayed due to crew oversleeping or negligently failing to show up.
Gate agents are underpaid and subject to the bulk of customer abuse and I sympathize with them. But for a gate agent to announce correctly or incorrectly that the crew overslept is undiplomatic and irresponsible. "The flight crew is delayed, we estimate their arrival at XX" would suffice in the instance.