I think you answered your own question with your question. In spite of the myth that a Benz diesel lasts forever, they don't. At 274K, there are many parts that are not what they originally were. When it comes engine time, a new camshaft will be in order. If you were to replace it now, you probably wouldn't be able to tell the difference. In a gas engine the fuel is mixed with air, the amount of this mixture that is pumped into the engine determines the power developed. In a diesel engine, it is the amount of fuel injected that determines power. Unless the supply of air is severely inhibited, the power will still be developed.
Keep changing the oil and otherwise maintain the engine until it is too tired, and deserves rebuilding attention. Enjoy it until then.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in