A driver does more than interface with hardware however, it must also interface with the rest of the system which is a moving target. So even full featured drivers may and often do still need work just to keep them compatible.
Hey, I think I am running on that $40 AMD Sempron and it is doing quite well for me thank you.
I do not think it is even that much that the Linux kernel and driver devs want to ignore older hardware, but there is simply a lack of people willing to maintain it. If their is no one to maintain it in an active and ongoing project, of course it is going to be considered cruft that has to be removed, as there is no one making it workable. That is basically the policy the DRM and Kernel developers seem to be enacting. Older radeons have a better chance of support simply because there are people willing to maintain the code. But in the case of SiS and VIA chips, no one seems to be stepping up to do it, so these drivers are under threat. I can understand the reasoning behind it, and if these old platforms still do remain valuable and are in need of further support, someone who cares must step up and maintain them. If not, you do as I suggested and just stick to older versions or use speciality distros.
Also, I am unsure of what I am 100% right on. If you are saying that I am 100% right in saying that someone could use older or speciality distros for older hardware when it comes to end of life, I would agree with you, if you are saying that we should always drop older hardware just beacues it is old, I would not. As I said, I love my old Radeon 9200 and continue to use it. And as long as someone is willing to maintain it, that code should remain.