Planes being ready doesn't mean you can leave. You need a plane and crew (and a few other things). Where is this aircraft coming from can help you see odds, but like relying on the seat map to judge loads of flights, its not too reliable. Aircraft can switch, crews come in from different places, weather can change quickly, crews can get sick, etc. Just as easily as the crew issue, the plane could have arrived with an MX. Where is this aircraft coming from can't predict that. Would you have been less upset if at T-24 minutes, there was an MX delay?

While they may have known about the scheduled crew coming in late, they do have reserve crews, or can stitch together crews from other places. So it could have been what you said - that they knew - or it could be rather than announcing a delay, ops were trying to get another crew together, which apparently, they did do. Unfortunately for you, it was too little, too late.

Because of the potential for delays, and not just UA, but whoever I am flying, I never, ever book to arrive just in time for a meeting or event, particularly if it is important to be there. I always give myself a few hours buffer between arrival and where I need to be somewhere. I've missed connections and had delays in the past for worse - and been forced to overnight.

I know its convenient to use the red eyes for a morning meeting out east, but your example shows the potential downfall. A single delay can screw things up - its a lesson learned the hard way. Next time, if the meeting is important - might think about flying in the night before. The extra expense of a hotel is probably worth it if the meeting is important.