Would've never thought of having a non-tech-understanding person use GNU/Linux before, but nowadays I think Windows has managed to close the gap on its own by becoming horrible. Though there are other options like iPad or Chromebook.
I had my ageing parents set up with a Linux desktop for some years in the past. I just -pre-configured everything they'd likely have needed. Browser, email, instant messaging (as it was at the time), a photos app, possibly something else I'm forgetting.
It's the somewhat tech-savvy (or at least somewhat tech-independent) for whom such a transition might be the most trouble.
(And, of course, those people who use specialized applications or hardware for which support may not be available, or who use workflows that require going somewhat below the "appliance" level and which could require significant relearning.)
GNU/Linux actually might've been best because it's harder to do anything in it. Chromebook wasn't an option cause the screens are too small, and idk if Chromeboxes were viable back then (~2015). She used to use Windows, which was an unmitigated disaster with viruses.
As for other issues with basic use, my mother is even older now, and even things that used to be pre-arranged and familiar have become more difficult. But I don't think the OS matters much for that any more, on a grand scale. Touchscreens might help to an extent (and also avoid upside-down mouse issues), and obviously things would need to be set up so that there aren't any unnecessary hurdles. But beyond that it's learned routines rather than any kind of a generalized understanding anyway. She does know how to usually get rid of things by clicking 'x' though.
Did you find something that worked out for your grandma in the end?
(edit: You edited your reply before I finished mine, so I replied to the earlier version. I see the problem with accidentally triggering things. I guess it might be the fairly cautious nature and the limited use patterns that might have saved my parents from them.)
Nothing worked. In hindsight, maybe the Mac would've been ok-ish if I'd used Chrome Remote Desktop to help whenever she had issues, which had fewer pitfalls than the other screen sharing options I tried. Last one I remember was Teamviewer, which broke cause her friend "fixed her computer by removing junk" like that, but it also had other issues before.
Screen sharing for support is probably a necessary problem I didn't think of much since I happen to live close by. It'd absolutely need to be something that requires minimal effort from them.
Sibling commenters have it right - if a computer is an appliance to them, then going for the simplest, least likely to accidentally mess up option is the best one.