Except Macs that go out of support do turn into pumpkins. Because software will start to refuse to run on it because the OS is too old. That is how I define unusable.
OpenCore Legacy patcher is awesome but it is not without issue.
I guess some people think that unsupported operating systems causing GPU glitches or being stuck in ancient application versions is a better experience. Not me. I am very happy to put Linux on a Mac, get better performance, and enjoy totally up-to-date applications. If I really need to, I could always run one or two macOS or Windows applications in a VM. I am not sure what those would be though.
The downvotes make me chuckle. You know, I actually did not really think about how much Apple hardware I had until I listed it off. The Apple fans and I obviously agree on some stuff ( just not the OS so much I guess ).
I do have an iPhone 15 though ( running iOS ). I am not very likely to put Android or Linux on it anytime soon.