Even if your car is from the same brand as the dealership, they may not be equipped to service it. At what point does that become reasonable? Just how old is too old?
For example, Mazda rotaries are infamous for being unservicable at Mazda dealerships, even when the RX-8 was barely a decade old.
The trick I learned years ago was not to bring my car to dealerships that weren’t around to sell my car new.