sooty
I would think that the AOHE is a backup for a system that at times, works too well, the Fuel. Thermodynamically, air is not efficient compared to liquid. Up high, where Ice is forming, the Oil runs cool, and the FOHE isn't as critical as Low, where Thrust demands are high, variable, and the Fuel is warmer. My picture of the Delta flight is high cruise, very cold temps, Icy Fuel Pipes, and not much need for Oil Cooling. So when the FOHE packed up, it wasn't in danger of being unable to cool Oil, but in causing Fuel Starvation. Had the phenomenon been addressed, there would have been an Alternate Fuel path available, sensed by diff. Pressure at the FOHE "ends". IMHO
AF