Better to have one system unlikely to fail than two dodgy ones one to catch the frequent failure of the other..
but that is exactly what we don't have - we have 2 excellent and reliable engines in most modern twins.
The difference comes when an engine does fail in a single compared to a twin - if you are over a nice landing area with plenty of height to set up your EOL then no problem in a single BUT when you are at high AuM at low speed, low level or over inhospitable terrain/sea then a failure in a twin leaves you able to get home, a failure in a single leaves you dead....it's that simple.
In the car analogy, due you want single or dual circuit brakes, can you accept the weight penalty for ABS, traction and stability control, airbags etc etc none of which are essential for driving but all make it safer if something bad happens.
This being shot at nonsense is just that - in Vietnam very few engines were hit despite taking fire often.
and you know this because you were there????? I reckon for the number of helos lost, quite a few will have taken rounds to all parts, including the engine.