Usually the end of life for a Diesel
is signified by hard to start (low compression) and excessive oil consumption
and smoke and sometimes down on power ( low compression), low oil
pressure when hot and idling is a sign of bottom end wear
Usually this occurs gradually, sometimes so gradually you don't notice, rarely is it a sudden onset type of thing. Excessive smoke is hard to not notice though and I have had farm Diesels than ran great, made great power with over 10,000 hours on them, but did blow blue smoke, especially if I let them idle for long.
Usually if the oil has been changed regularly the bottom end ( crankshaft, rods etc ) stays in relatively good shape.
Once in a blue moon an oil pump fails or other catastrophic failure occurs, but that is usually very rare, and truth be known can happen to any motor
at any time.
As they get old age wise, number of years, they may begin to leak oil as the seals
break down, but my Yanmar
will hit 30 next year and doesn't even seep oil much less leak any, so that is not always true either.
Its almost like asking how long will I live? Some make it past 100, many don't make 50, but how an engine
is treated will I think make it last a long time, or kill it early.
I have never, ever worn out a larger engine, I have run cars over 250,000 miles to where the car was falling apart, but the engine was still good, and I see many driving around with 50,000 miles on them and smoking.
But I have always wasted lots of money
on the best oil and filters too, I probably pay twice as much for an oil change
I do myself than I could have it done at a Jiffy Lube, but I use the best I can find, they don't.
I just sold last year the International Harvester 575 farm tractor that tried to kill me when I was a kid, looked like the devil, covered in rust, but started and ran great.