I’m using the same iMac I bought in 2011.
Apple has updated the operating system twice and so far, there have been no bugs, no attacks, just smooth sailing.
There are no Apple stores hereabouts in the Ozarks. I bought the machine directly from Apple and had my son (in Seattle) walk me through via two or three phone calls. He spec’ed the machine and I’m grateful for that.
Good for you. As long as you are happy with your machine, and it does what you need it to do, don’t let anyone tell you that it (or you) needs to change.
If you feel an overpowering need to purchase a new machine and play with the latest OS, then of course feel free. But if it already just works, do as so many other do and don’t try to “fix” it. It’s not like it is broken, after all.
Cheers!