I thought the whole idea behind a "cool-down", from what I've read here and there over the years was a means of active recovery.
For example, if you've had an intense anaerobic exercise session, some gentle cardio - anything that gets your blood moving - would help move metabolites out of your muscles a bit quicker. Such gentle cardio can even be walking. If you took a 5 min walk to the gym, your walk home would count as a "cool-down".
As for the whole heart attack if you don't cool down, I believed it was a perceived risk for those with a lot of muscle bulk. In a pumped state, a lot of your blood is filling your muscles, leaving less for your heart to pump. This is purely anecdotal.
The heart always has the same amount of blood to pump its just that blood is used differently when a muscle needs it. To work a muscle needs oxygen and the blood brings that to the muscles. The blood doesn't stop and hang out conversating with the muscles its still returing and continuing the oxygen exchange in the lungs. I remember reading correct me if im wrong that the pump in the muscles isn't completely blood its...blood plasma?
Got me thinking I will research this later.
Homeostasis and the body is a wonderful thing. We sweat to bring our body temperature down back to normal. We breath heavy because our body (muscles) needed the oxygen. I think after doing some of my studies for school I believe that cool downs are not needed the body does it naturally. I would say that if some people that have heart attacks or issues after a workout that was a condition previously undetected. Always always consult with a dr.