In my area, there are two kinds of W123's. One is the kind that was owned by the quintessential older Mercedes owner who might have bought the thing new, has a four inch thick folder of receipts for every scheduled maintenance item and who fixed every little problem as they arose.
The other kind might have once been like the above, but maybe 5-10 years ago was bought by some younger person to do a fry-oil conversion. Either they did or didn't, but they drove it around for years and neglected the maintenance and they've just let things like sticking locks and broken instruments slide. I've certainly known people who've done good fry-oil conversions and took care of their cars, but I've also known many people with more environmental zeal than mechanical aptitude who've bought W123's (and other older diesel cars) to do fry oil conversions who end up just driving them around like any old disposable econobox.
I every now and again find myself looking at one of these old Benzes and I've found that the condition of the car is almost directly proportional to the age of the owner. Part of what used to be so great about these cars was that not only were the drivetrains invincible, but the people who typically owned them took extremely good care of them. Certainly the march of time is part of why there are more junky W123's out there, but I think the change in owner demographics is as well.