What makes you think "Grandma" will be installing anything? Are you seriously comparing installing a complex OS to just using one already installed?
This isn't about installing an OS--this is about using one.
If that was true Windows would not be owning the market.
I write computer software FOR A LIVING.
I am fully aware of how vastly superior Linux is, as an operating system. But we have to use Windows to get anything done. Programming on Linux is like a trip to the 70’s