A few things:
1) Looks like NYC. I was raised in NYC and I can tell you it takes one wrong decision to mess with the wrong person to get stabbed or worse killed. This one time, in my younger HS days, I got in between this random drunk girl and a man cursing her out (who looked like someone she knew). And that almost got me killed. Not only that, but even the girl got mad at me. I was dumbfounded, and yes I pooped my pants a little, not gonna lie. I was in HS and this was a grown buffed guy.
2) I am obviously not justifying this man's actions. We should know better, men are physically stronger so it will never be an equal confrontation. But without having some proper context and full convo, and unsure what she was telling this dude, but assuming this is a modern feminist that usually will disrespect men and get physical/confrontational with men, when a guy gets in their face they feel like they can take it just like a man. What they don't realize is that men and females are different, but they are still in the magical clouds of absolute equality. Some men have a short fuse and this can happen. Especially if you trigger someone.
3) And on that note, she took it like a man. Wow, that looked like a solid punch from a pretty big dude. She took it. Respect.
To your point, I do feel like people around could have de-escalated the situation. But I mean that is the society we live in and shows even more in a city like NYC. I can tell you from experience, crazy things like that happen in the subway all of the time. People are simply desensitized so I am not surprised. But if that was her bf next to her, then yes that is quite surprising and alarming he did nothing.
Modern Man Advice