I wonder whether it is simply that the mechanics working on these cars are too young. 1985 W123 is now 21 years old.

Now this might ruffle some feathers, but I will say it anyway. In general, there of course are exceptions, people who become auto mechanics were probably not exceptional students in high school. My experiences have been that often times close to right is OK with them. Examples are peripheral parts not re-connected, oil overfilled, transmission fluid overfilled, tire pressure not to spec at tire shop, dings in alloy wheel at tire shop, etc etc. It is also true, in my opinion, that during long days of twirling wrenches under pressure to get the work out employees dripping sweat and covered in dirt just may not have the same caring attitude and attention to detail that we mechanic hobbiests possess.

I just had about $1600 worth of work done to one of my vehicles, 2001 Voyager mini-van. I have no interest in working on it. So after transmission service it doesn't shift like it did before. There is a loud squeak in the rear.
They did something to adjust the shifting problem. The squeak was a defective new rear shock. Point is that they never test drove the thing.
This shop is owned by a very knowledgable honest guy. However, he can't watch over everything that the mechanics do. the other repairs were done satisfactorily.

This is why I go behind and check what I can to try and catch the screw ups before harm is done.

Steve