As an engineer, not even one with engines as a specialty
As an engineer with 36+ years with turbine engines as a specialty....
Destroyed may be a bit strong, I probably would have said "trashed", but destroyed is not that much of a stretch.
Even at idle, a 777 fan is turning something around ~600 rpm (I'm guessing Trent here, it would be a bit lower if it was a GE90). At that speed, the impact with a zero rotation ULD is going to rapidly result in much of the container turning into aluminum confetti. The portion of the ULD that survives will bounce into the inlet/fan case, loose rotation, and go back into the fan resulting in more confetti, repeating until the ULD is, well, aluminum confetti.
That confetti will be flung into the fan nozzle/thrust reverser at high speed, doing considerable secondary damage.
If they are really, really lucky, that metal confetti will be centrifuged enough to miss the core inlet. Likely result - a set of fan blades, fan exit guide vanes, inlet, fan case, and thrust reverser will need to be replaced (some bits possibly salvageable, but with major repairs needed).
OTOH, if bits of the ULD went down the core, many of the compressor blades are bent or broken. Best case, secondary damage is minor and limited to the compressor - worst case multiple compressor blades broke off and corn-cobbed the entire core (compressor and turbine).
So, in short, the best case is $1-$2 million in damage, limited to the fan, fan case, and nacelle. More likely, if bits went down the core, the damage is extensive and it could easily be that the engine will cost more to repair than it's worth (sort of like a car - build one from spare parts and a $25k car would cost over $50k) and the undamaged bits will become spares.
BTW, absolutely no reason to replace the other engine. Operators routinely used 'miss-matched' young and old engines on the same airplane.