My boss has an 84 528e (I think) w/ 273,000 miles and it runs like a dream. If you maintain the car it will run for as long as you want. Most people trade in their bimmers because the "other" maintenance becomes too expensive.
The other reason your car might feel like it's been running better is because you might be taking better care of it, i.e. using better gas. Higher octane gas and regular oil changes will really help to clean out an engine.
I know exactly how you feel though. I love these cars.