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.
After 5 or 10 years is when I’d start looking for a different mechanic, but if it’s a generation that lasted a long time then possibly more.
Dealership techs are used to working on new cars, and used to the issues that new cars from their brand have. I want to take my car to people who are used to the issues that cars like mine have.
My 1991 Suburban has much more in common with a 1982 F150 than it does with a brand new Suburban.