I have the C-Class version of that diesel. With all options, it is a very refined quality vehicle. Like all European high quality cars, as in Mercedes, BMW, and the like, they are so well built they do keep going reliably for a long time, which is a situation the makers have to live with when pitching their product at fleet lease renewal time, especially to Munich taxi-drivers. Once past a certain age, they depreciate faster.
The price of dated spares, and restoration work, will compete with the depreciated value of the car. I just keep fixing up mine, and keeping it serviced, sometimes with good condition reclaimed parts, so I can keep driving it. It is by far the best car I ever owned, but I know there will be the day when the cost of a spare and fix is way more than the depreciated value. It becomes more valuable to me as a thing to keep driving than what I might get on a trade-in.
There is a turning point where it maybe becomes a desirable classic, and will start to appreciate in value. If it is in good condition, runs well, and by self-skills, you are able to by-pass some of the repair costs, say by taking advantage of reclaimed parts, then it can be more valuable to you as a car to just keep driving, and enjoying.