It's not so much that. I had switched to Mac in 2007 and got them to switch. At the time it was much different in terms of viruses, and that machine ran smoothly until 2018 (I added ram and swapped HDD to SSD along the way I think).
I had them upgrade last summer due to their old machine no longer receiving updates, and ever since it's been hit or miss. I guess they can do more on the new machine because it is faster?
I think a good Windows 10 machine that is locked down would be solid, but at 84 transitioning back to software you haven't used since you were 72 is a challenge. They've had the Chromebook about 5 years and still haven't learned it all (not that there is much to learn, but they still aren't clear on how webmail on chrome lines up with the mail client on Mac, or how to get to the Gmail web interface from the Mac, even though they've done it a few times (and I've shown them). I convinced them to get a Chromebook so they wouldn't have to take the iMac with them while traveling and it would be easier if it were lost or stolen.
I would say it's time to just quit the internet and computers, but it's part of their freedom that I think they'd like to preserve and they do use it for light document editing, viewing photos of great grandkids, etc.
I guess I'm looking for guardrails / bumpers that I can put up to keep them on track. And honestly if this problem was solved in a non-patronizing way it would probably be a very successful security company that could be applied elsewhere. At the end of the day it is least privilege and fighting adware / malware / all the crap that most users run into who don't have good security hygiene.