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.
Cars aren’t exactly cheap new or second hand, I don’t think it’s asking too much that you could expect you’re car to be fixed at a dealer at any age. Obviously you are still paying them to fix the car but the option should be there.