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.
My experience is that you really don’t want to take your older car to a dealer. My last experience taking my 2000 Porsche to the expert” Porsche dealer was a disaster and I ended up fixing it myself. I only took it to them because insurance was paying and insurance won’t pay you to fix your own car.