So I am not an emergency worker, and I need that to be clear because my sample size is small. I do work at a very large anime convention, so I've been around a few emergencies. Not many. But enough to notice a pattern:
The problem is not bystander apathy (which is not a thing), but bystander confusion. There's this moment where everyone is sort of processing "fuck, that just happened, now what" and loses the ability to think logically about next steps. As soon as one person steps in and says "you there! Do this next step!" the confusion breaks and people begin to react to fix the situation. Literally all it takes is somebody basically going "yep, we all saw that, now let's move."
While the murder of Kitty Genovese was a bullshit homophobic coverup, if you're in an emergency situation where someone needs medical assistance and there's more than one person, you can help a lot by loudly but calmly saying "Does anyone know first aid?," giving a chance for someone to respond, and then singling someone out by something identifiable--"hey, you in the red shirt and Steelers hat"--and telling them to call 911 or your country's equivalent. Even if nobody there knows first aid, as soon as someone sounds like they know what they're doing people will start trying to help.
Also a few things for you to know that I learned many, many years ago as part of my Red Cross first aid training and haven't forgotten:
1) stay calm and encourage the injured person to stay calm.
2) for a stroke, ask the person to raise both arms at the same time, and to smile. If you observe unevenness in either motion, call emergency services IMMEDIATELY. Seconds count.
3) the "cough to stop a heart attack" thing is a myth. Call emergency services immediately and have the person sit down.
4) contrary to what Sherlock would have you believe, you should actually remove belts, suspenders/braces, and tight clothing from an injured person. Yes, if the injured person has breasts this may mean unhooking their bra. Now is not the time for modesty.
5) never, never give a severely injured person water unless instructed to do so by emergency services (spoiler alert: they're basically never going to tell you to do that). It can cause vomiting, and then you have more problems.
6) never move a severely injured person or someone who's hit their head even if they "seem fine." Your kind intention may kill. Wait for emergency services and keep the person calm.
7) never try to restrain a person having a seizure.
8) only move a person having a seizure if they're in immediate, imminent danger (for example, falling right next to a campfire they may seize into). Banging an arm into a chair leg is not imminent danger.
9) "swallowing your tongue" is a myth. Never put anything in a seizing person's mouth.
(Can you tell I had a classmate with epilepsy?)
10) bleeding injuries should be raised above the heart.
11) I'd also encourage all of you to get your Red Cross or some equivalent certification. I actually used mine for the first time about six months after I got it, and it did in fact save a life. (And you will notice I said "for the first time." Most of those incidents weren't nearly so dramatic--99% of the time it's been me saying "you there, call 911" and rolling somebody into the recovery position--but every single time I've been damned glad to have it.)