Considering MB still has the parts for 1960's cars and even earlier still readily available, I think the W115, W116, W123, W126 and W124 all have quite a long life ahead of them. At least another 20-30 years before they are considered pre-historic. I bet the biggest impact will be a fuel changeover or a massive spike in fuel costs to $6+ a gallon, so people buy extremely small efficient cars like in Europe.

The cars will indeed go forever with maintenance and keeping the rust/tree hits away from them. My W124 is almost 20 years old and has virtually no rust, only 142k on it, and runs perfect.....all it needs is a bit of suspension/steering refresh, and it will be perfect. All for about 14x less than a new E-class would cost, not to mention the other expenses (insurance, registration....etc....)

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)