If they keep updating a 4 year old phone, they will be doing it out of their pocket. They are here to fill their pockets, not empty them. First they build aosp for the phone, losts of testing, relicense all the services and drivers for lastest version, get drivers from their HW manufacturer, lots of testing, test failed? Redo everything or do not update. Passed? Put custom skin. Test more. Now this takes a lot of resources and a lot of money. Not sure how many years they can do it once you pay them 30k.