Apple does for the sole reason of forcing sales. For many the latest version of macOS will run perfectly well on older HW. Thankfully the community acted and has solutions like
OCLP.
Apple's talks of being green, but is just that "talk" as the moment such matters impact revenue it's all off to the landfill. I own and use both current Mac's & legacy Mac's and see no need to replace until they no longer serve purpose.
If Apple was serious about it's concerns for the environment it would conduct itself in a very different manner, it doesn't plain & simple...
Q-6