I'll go the other way and say there should have been 18 stories within 90 minutes. That would be the only way to experience the true horrors of war, removed from the human characters etc. The prologue of Battlefield 1 actually did this brilliantly, where each time you would die, and it would transition to another soldier, so on and so forth. I don't believe war dramas which focus on one character or one story is faithful to the true nature of war, as such films end up being about the success of one character, rather than a combined failure of everyone involved. There are some movies like that, such as the Hungarian film, The Red and the White or to a lesser extent Full Metal Jacket.
Nevertheless, this is a great step forward for big budget Hollywood war dramas. I loved that Dunkirk didn't have a singular focus on one or two characters, and didn't bother with developing them and getting us to sympathize with them. It just moved on from event to event, from character to character, to give us a little hint of war is like. But most of all, I was floored by the use of real aircrafts, real sounds, real stunts, without any CGI. Brilliant on IMAX! Only Christopher Nolan could have raised $150 million in financing for such a film. Every other director would have been asked to add character development etc.
The dog fights were absolutely amazing! In fact, all of the visuals and sound were.
I enjoy both types of war movies that you mentioned (individual stories as well as nature of war). For me, it just depends on the movie. I personally would have enjoyed this one more if it focused on the individuals. But I can see why others (including my wife) strongly disagree with me. I really wanted that huge emotional payoff when the civilian boats arrived. But war isn't like that. It was my fault for having a predetermined expectation walking in. I think if I had never seen any previews or knew very little of the story, I would have enjoyed it more. (Although perhaps "enjoy" is the wrong word.)
Without character development I find myself being rather desensitized to just nature of war. 18 stories would mean nothing to me at all. There is just no reason for me to be emotionally involved in any of the stories at all. Furthermore, I feel with each story only lasting 5 minutes even if I had any emotion toward first two or three stories by the end I would have left being bored of it.
With that said this movie would just not been for someone like me. From what @hanshotfirst said about not even knowing the main characters well. I feel your method at least would push the envelope in one direction. When creating something out of the normal experience perhaps 18 stories would create that shock and awe feeling of how brutal and short lived life was on the battlefield. Either way, the movie just didn't do enough to meet either of these types of views expectations.