Most Chromebooks have Android applications now. So it's more like a browser and a tablet on steroids. Many newer ones (even cheap) have Linux support.
But of course as you said it's a game of trade offs. My father was using Windows years ago and because of constant service needs I set up Linux for him. It was easier for me to service it remotely. It was better, but far from perfect. Later on I bought him a new computer that had a Windows license, so with upgrade to Windows 10 I thought that it would be ok. My father wanted Linux back after a month or so. With newer distribution and newer computer it was much better than previously. Now it's pretty alright.
However I still think of something a bit more like ChromeOS. Then it would be quite maintaince free. Not Gallium, but a full distribution with same update model as ChromeOS.
I.e. a web browser with a keyboard.
> an iPad
I.e. a content consumption tool, not a general-purpose computer.
> a MacBook
Fine if you can afford it.
> or even installing a Linux distro and setting it up for them such that they don't need to use the CLI to accomplish the things they want to do.
That's... possible, but tough. As someone who switched from Windows to Linux many years ago, I still see every distribution to be "leaking" the fact that it's a CLI-oriented ecosystem. They do that that especially when someone goes wrong.
All are valid alternatives if you carefully consider your parents' needs and discover they're extremely limited. But Windows would still be my first, default choice - because of a combination of its design and history, it's the cheapest commercial system that still lets you own your computing experience, and run professional software.