And I don't see why they would be frustrated. Be polite and let them know when you are leaving. No big deal to us.But if they are knocking anyway then the door sign is being ignored. Offering some indication that you don't want someone to knock your door has been standard at all hotels for a very long time, its one of the most basic things in every hotel of every class.
I don't see why being frustrated about such a basic of running a hotel is immature?
If it does upset people, then they are free to talk with the hotel manager. As you said, no reason to take it out on the person at the door.I don't think the anger should be taken out on an individual, but more whoever made the decision that do not disturb or room occupied signs should be ignored.
So if checkout time is no later than 11am, I fail to see how someone would still be sleeping at 8:30. Again, I understand that's us, and people may like to sleep in, but if you are showering and packing that morning, you should be up quite a bit before that. We don't like to rush on any morning. JMHO.If a knock is at a reasonable hour then I would be polite and let them know what time we might leave. But if the room occupied sign is on and a knock wakes me up then I think a complaint is justified.