There's a lot of uses that still require a real, local desktop, and I think once you give this to all of your non-techy family members you'll start to run into them.
I don't understand why those complaining about maintaining Windows don't just put on Ubuntu. LibreOffice is certainly more familiar to Office users and more compatible with existing Office documents than Google Docs.
If you haven't considered Ubuntu yet, why do you consider Chrome OS? It's not very hard to teach even the least technically literate person that they just need to click the big multi-colored circle to get to the internet, so it seems weird that abstracting that one step away makes everyone go wild about giving their mom a Chromebook.