ChromeOS is a far better "Grandma" distribution than Windows or MacOS. Linux is simultaneously both the most Grandma friendly and least Grandma friendly, including every point in between.

You need to compare apples to apples, aka pre-installed vendor supported operating systems. If you're comparing installing an unsupported OS on unsupported hardware, Linux is far superior. Try installing MacOS on a Windows computer or Windows on a Mac if you're truly masochistic.