I've spent some time studying your equation model of car expense satisfaction and must say that I found it intriguing. What it fails to take into account, though, is the depreciation-driven downward pressure on purchase price for older cars, that is not matched by a downward curve in parts cost and labor expense. A mid 80s MB diesel can be purchased for a couple of thousand dollars and yet require $1000-2000 a year in maintenance, depending on miles driven. The reason is that labor rates are the same, no matter the purchase price of the car, and the parts are still priced to reflect the fact that the car was a $30k car when new.
To end the discussion there is to miss the point, however. Ask any MB diesel owner and they will probably tell you the same thing. They probably cover more miles in that car than if it were a gas car, their cost-per-mile is probably much lower than if it were a gas car, and if they sold their car they would probably tell you that they miss it and it was the best car they ever owned. Part of the reason for this is somewhat intangible...the feeling of utter safety, knowing that you are driving one of the strongest cars on the planet and one which will effectively sacrifice its life to save yours; the fact that the driving position is the most comfortable you've ever felt (my 123 is much more comfortable to sit in than my 126 of the same year!); and the sense that your car is similar to the Energizer bunny in that it keeps going and going. These are important factors that do not fit into your equation.
The way I've always looked at it is that my average monthly maintenance costs were less than a car payment, and I was able to pay cash for far more car used than I would get new. Except for AMG cars, current production MBs are being built to a price, not to a standard. The older cars were built the other way around, and that doesn't fit into your equation either.
The equation is clever, but works best with commodity vehicles. Older MBs are not commodities and the value owners place on the cars is as much intrinsic as extrinsic.
'76 Moto Guzzi 850 Le Mans