Aftermarket is not the dealership. Yes you can get aftermarket parts for a vehicle after the company stops making them but they won’t be available from your dealership.
You said explicitly the dealer/manufacturer would still provide parts and that is just not true.
I can get oil filters, some engine parts, light bulbs, oil and many other parts form dealers. Some sheet metal parts are available at a dealer for quite some time. Suspension parts are also available for longer periods that software support. I did not say that all parts are available form dealers/manufactures.
You said explicitly the dealer/manufacturer would still provide parts and that is just not true.
I did not see that on previous posts. Some parts are available for quite some time.