I don't think we are. Orgrimmar didn't deploy it's whole garrison. At least I don't remember reading anywhere that it did, I just know Saurfang needed time to muster an invasion force. A rule in warfare concerning defensible positions such as fortified cities is that you need considerably more manpower to overrun it than defending it. That's why siege is often preferred. SoO was a case of champions rushing the city, which was just Blizzard once again deploying rule of cool. If Warcraft was based in sound military doctrine "Siege" of Orgrimmar would've been an actual siege (in which case it couldn't have been a raid). Usually rushing a city is chosen over siege when time is of the essence, e.g. winter was often a major consideration in real world history. Had Alliance formed a siege (against Cold War principles) Orgrimmar could've probably held until the Silithus-bound forces returned, or another force from Silvermoon/Undercity/Thunder Bluffs/Suramar/Highmountain rallied to help. There's no way War of Thorns was a case of all eggs in one basket.
No, quite the opposite, it was the site where the western Allied forces would enter and the Germans knew it. Google "The Atlantic Wall". Germany had fortified the entire coastline facing Britain. An immense feat, but the landmass was so vast they had to focus the brunt of their defenses to where they thought the brunt of the invasion would happen, and thanks to Britain's misinformation campaign that was declared to be Calais. If I recall correctly it was Erwin Rommel himself who oversaw the operation, though he had to share his position with another less known general, who had his own opinions (Side note: Hitler liked to appoint his generals in such antagonistic positions, so that they would enter a rivalry, supposedly to increase efficiency. It didn't really work).