Reason for my thoughts are that a tuned engine is more likely to be driven harder and most of the chip tuning/remaps are taking the engine closer to it's deisgn limitations - I'm no expert on diesels but I understand the exhaust gas temps on these can rise as the turbo works harder etc, which would surely lead to a shorter life?
Whether any reduction in durability would become noticable or cause any problems during the car's lifetime is another matter - very few engines 'wear out' these days anyway.
If it's done correctly and not 'over-tuned' then it shoudnt really effect the lifespan of your car's engine!
Your vehicle/engine is perfectly happy with additional Torque and BHP however if you go over board and give it too much power then you run the risk of putting added strain on various components. Your clutch, gear box, turbo, injectors, tyres and fuel economy can all suffer as a result of trying to extract too much power from your vehicle!