In aviation diesel is still perfect as it can run on kerosene that is cheap and available everywhere. Avgas has become exotic in many parts of the world including the military's.
However I'm still not sure if the perfect aviation diesel engine already exists? Not sure about modified car engines. My choice would be nothing that needs battery power to keep going.
For private car use I see diesel engines dying. They have been used for the wrong concept: Instead of making them maximum efficient (that would have been the logical choice) they went the maximum vehicle size plus maximum horsepower/torque route. Not being as clean as advertised will kill them now. You can see the sales numbers erode already.