The problem is that rather than offering a solution that didn't require changing everything (of which there are plenty), your recommendation is to change everything.

You may have migrated people to Linux, but that doesn't make it the best response to every problem.

People had a hard time adjusting to windows 8. People still have a hard time with windows 10 in some cases, even though it's much more in line with what they're used to. Changing OS means the options and menus they are used to and know how to find are now somewhere else based on different logic, worded differently, missing, or more complex. This woman doesn't need to relearn how to use a computer.

Additionally, local techs may not work on Linux and there's no 1800linux number for help for grandma. It's forums and such that she may not have any experience at all with, meaning even more to learn just to get where she already was before you changed the OS because of one recurring upgrade reminder.

Even further still, win10 will never be mandatory. It's a free upgrade for a limited time, and a different OS, not just an update. The popup can be stopped by preventing a single update or installing a single simple application. A different OS is a hell of a step further than a single application.

EVEN FURTHER STILL trying to force your preference of a more advanced (to your average bear) OS on people with minor problems just makes you seem out of touch with the actual public user base. People want familiar. People want the same as they already bothered learning. They don't want to be told to alter their understanding of the basic operation of a computer.