I think that for OS X (now known as macOS), as time goes forward, that the newer hardware gets all that --- well, new hardware. And, the operating system needs to support that, with the various drivers and other support libraries that are needed for that support.
Time goes on, the new hardware gets the attention, the older hardware often continues to work, because the new drivers still support the older hardware, or the older support remains in the system.
At some point, the decision is made to not test, and therefore not support some older generations of the hardware.
The new system is near release, and engineers can determine that either the old hardware won't work at all, or may work with certain limitations. There has to be, at some point, a decision made to prevent installing on certain older systems, because the proper support just isn't there. That hardware frequently is the video/graphics chips in the system.
Some folks try to modify the system installer to boot and install on older systems, regardless of the issues. Sometimes those issues will be not noticeable, or simply makes the system a little slower, or might even be less stable. Apple, I think, tries to avoid those issues, where possible. The result is that older systems get dropped. Sometimes for an entire processor generation, other times, just because of video support.
And, as most Macs do NOT have upgradeable graphics chips, there's simply that point where you get to decide if you want to continue to use an old system, and perhaps miss out on security updates, or your own requirements mean that you have to upgrade to a newer Mac, because of the OTHER software (and hardware) that you use needs the new operating system.
Your nearly-9-year-old system is pretty ancient tech, believe it or not. And continues to work well enough, subject to the limitations that might exist in software that is not quite state-of-the-art.
I have a 2004 eMac that is in daily use. I use it for burning copies of CDs, used for a variety of music projects that are part of my hobby and my life. An internal tray-type DVD burner is much more efficient at that, than using something newer with an external USB optical drive.
I like to say that a computer only becomes obsolete when it will no longer do the job that you need it to do.