The engine on a modern diesel, should, if looked after correctly last forever, but due to the emmissions laws, and how tech has evolved, normally all the electronics, and exhaust cats, dpf, egr related items fail regular. An old diesel engine is far better for mixed driving, unfortunately its just how the law is now days on emissions sadly
This is true but having said that a lot of manufacturers have refined their petrol engines right down now to get pretty similar MPG figures to the older diesel engines.
Really diesel isn't worth having these days unless you do about 16k+