I feel i could positively contribute to this discussion. I am a tech at a vw dealership and I can say that the mechanics dont get to choose what we work on. Its whatever rolls through the door. Oldest car ive worked on so car is an 89 but we get beetles and buses
rolling through on a decent basis. They go to the guys who are the most familiar with them. And yeah when the 91 jetta rolls in you get the moans but we all write it up just like an 11 jetta gets written up.

In regards to not knowing how to work on them (again this is in relation to vws but its the same thing with mercedes) Most any tech working in my shop either has worked there long enough to have worked on them when they were new enough or have owned older vws themselves. Most of the older stuff is much simplier and any of us could diagnose it.

I feel bad that you guys are in places where people are refusing to work on your vehicles. I would never refuse working on a car because its "too old"