The problem with this is that you need to make those updates something people want to buy. And at some point, you have a game of diminishing returns. Even the hardware is starting to feel that way.

Yet, security updates aren't "free". I'd guess that keeping an OS secure is probably more expensive than developing new features. That part gets tricky.

Also, supporting 5+ year old hardware gets harder since that hardware stops getting made and the new stuff gets more and more different.