Interesting discussion. I'm not a pilot, but I am an engineer with experience designing commercial aircraft propulsion systems.
There are extensive fire detection instrumentation systems inside the nacelle of a commercial aircraft like a 737. But most nacelle fires are due to severe engine damage, and they are often fed by oil or fuel leaks. It is difficult for the fire suppression systems to extinguish these fires.
On the other hand, the engine pylon and engine systems are designed to survive largely intact for a 20 minute exposure to a nacelle fire.