In my opinion everything after Windows 7 progressively got worse. Windows 7 (with some exceptions) was about as good as Windows got.
If their computer is secure and they’re still happy with it, I’m not sure I would poke that bear. I’m glad that newer versions of Windows aren’t a consideration.
Be careful with that thinking though. I agree that there are many cases where people would be better off with a Linux distro, but after that switch EVERYTHING that goes wrong on that computer (small or big, due to the switch or not) will be pinned to the switch itself.
More than likely, you are signing yourself up for more computer responsibilities, not less in the foreseeable future. If you and your parents are ready for that, then now is a great time to switch.