I'm sure Samsung's logic goes like this...
When a phone becomes end of line, consumer will buy a new Samsung phone.
In fact I think they rely on this as these days a high-end phone from 2-3 years ago is quite capable of pretty much anything a new one can do. The camera might not be quite as good, and it'll be a little slower, but there really has been no "must have!" feature added in the last 3 years. In fact the old phone will still have a headphone jack! (Not so applicable to Samsung as they seem to be one of the few clinging on!).
The only thing that got me to upgrade my ageing Nexus 6 was the battery giving up the ghost (and every replacement cell I found looked like a Chinesium fire risk!). Apart from that it worked perfectly.
Consumers need to get a little smarter. Look at the support track record of a company before buying a phone. Only then will OEM's like Samsung/HTC etc etc take updates and after sales support seriously.
(Yeah, I know, fat chance!).
Biting the hand that feeds IT
