I know of at least two cars with 800k km with original engines. Both GM small blocks (Gen 2, multiport fuel injection so computer-controlled). Neither engine has been opened since they rolled off the floor in the 90s. They’re not particularly efficient (only about 270HP out of 5.7L) but if taken care of, they probably will go forever.

Definitely an exception, though. Very little else on those cars is still original. But it can be done.