Name factors that lead to demise (end of service life) of a 300D W123..
 

Yes, believe it or not, a 300D can die if an owner wants it to. Otherwise, it can obtain immortality...My feeling based on 9 cars in various junkyards, found without serious accident damage is:

(1) Engine/fuel pump/won't run
(2) Rear end /differential went
(3) Its never been rust

My personal feeling on (1) is the chain went or the Fuel pump got so out of calibration, the motor became unmanageable

Never saw one witha Bad transmission!

Rust, even here in Florida. I saw a 240D with the seats falling through the floor pans.

I do know that several cars have changed hands because of leaky old style primer pumps. The pump leaks air in, the engine won't start, the owner doesn't know any better and sells the car for a pittance.

I often wonder what does these cars in. I see undamaged W123s all the time in scrap yards, and I hesistate to invest in any drive train parts. I did get lucky with a 2.88 rear end from an otherwise undamaged 300D, but I also bought the turbo, and it doesn't spin very well. I suppose a bad turbo could send a car to the heap, just because of the repair cost.

A T-boned W116 I saw this past weekend was already missing the entire driveline.

I have heard that the turbo is indestructible...
 

and the engine is a very very strong. Mine ticks like a rolex even after a loss of oil to l.t. 1 quart and being run at 85 mph for 30 min with no water pump belt. The achilles heel is the Fuel Pump, which if it gets bad fuel or whatever, cah really turn a great engine into a dog.. some less than informed owners tend to try evrything but replace the pump...

Well eventually they just get tired. Everything needs work the suspension, interior, exterior, engine, trans ect. Rust also plays a major role. The 420SEL I parted out had a pretty good body but their was some major rust underneath. Also everything was just shot it was tired.

Yes I guess you can do the money pit thing or elbow grease thing
 

and work on it until you keel over from exhaustion. Or throw tons of money at it and have someone else do the work. I wouldnt rebuild the engine now, nor the transmission. I don think I would do that on any car anymore, there are too may used cars outthere for $1k that you could easily fix..