I wonder how many of these runner death from fit people are the result of not recognizing something is amiss. Yes, I can picture some rare cases where you have a massive clot that breaks off and it goes from good to really bad super fast...if that's what all of these are, then it's just bad luck and not much you can do. Perhaps imaging for the arteries to see how they are but outside of that, not much.
I wonder though, how many of these guys that end up dying running/cycling did have signs beforehand. Maybe just not feeling well, some odd pains, etc. If you're close to a heart attack, then running can definitely be the thing that tips you over the edge. That's obviously a massive stressor and if your heart is working hard just to barely supply itself kicking up the demand can be overwhelming.
I think there is also a tendency to report arrythmias as heart attacks, so that also has to be separated out. Not sure how much can be done for arrythmia though in terms of screening. They can catch some, but plenty they can't. Pretty rare though, so without any reason to suspect it I feel like you have to choke up the possibility of dying from SCD running as a bad luck anomaly that just happens if you are unlucky, in the same way sitting at a red light and getting smash by a truck that couldn't stop might be.