I would classify the Dunkirk-haters into three categories. There is simply the anti-war crowd, and they see this as patronizing war or it’s associated violence. The second crowd are the type that generally want some romance piece out of any epic story (War and Peace, or Doctor Zhivago for example). The third group are the precise-history crowd (A Bridge Too Far is a good example of this group), who want every single detail and name to be absolutely precise.
For the bare essentials individual who really doesn’t remember much about this topic in school or college...Dunkirk is a reminder of three essentials of life.
1. If you say something is impossible...then you ought to go and put every ounce of strength into proving the statement is false.
2. If you’d lined up various nationalities on the Earth and just said we need civilians to haul up tomorrow...manning sailboats and pleasure-boats....to face down some Nazis and rescue some guys in uniform, then there’s only two groups on the face of the Earth who’d do something this crazy (Americans and Brits).
3. Finally, there are absolutely consequences in life. Without that rescue, the Nazis likely come to England within six months...invade....taking the isle. That one screw-up, and the dynamics of rescue...set about the consequences in five years of Berlin being surrounded and defeated. Consequences matter.
There were two or three world-changing events in the last century, and Dunkirk (and the time surrounding it) was one of them.
For instance, Guderian halted his Panzers, allowing the Brits and French to reinforce Dunkirk and the escape to happen. If he keeps pushing, it's likely that the pocket is more or less annihilated, and the Brits sue for peace. Or, are wiped out themselves. Either way, America and Germany likely arrive at a negotiated armistice, with America probably getting the worst of it.
IMO, and there's always room for discussion, Dunkirk was the moment in time that irrevocably changed the direction of world history.
I like your part about Americans and Brits. The more time goes by the more I like the Brits because I realize more and more how much of our basic outlook came from them. Just have an increasing sense of the heritage.