This is fair. But at that point the same could be said of a Chromebook for her needs, which I’d venture is true for most people’s computing needs given entire swaths of the world do everything on a phone or tablet.
The Linux vs Windows debate is peculiar, because it really only applies to users who are more advanced than the average, arguing about problems that only arise when you want to do more demanding things with your machine like development and gaming. Your average user doesn’t care about any of the anti-monopolistic / FOSS reasons to use Linux, which makes the argument for them essentially “you should use this operating system that takes more work to use because it’s better for you for reasons you don’t care about.”
In order for Linux to become more mainstream, it needs to be able to exceed Windows’ performance and ease of use for gaming and productivity - which is challenging since when most users think of productivity apps, they only think of Microsoft products. It’s not enough to be equal in order to compel people to switch from what they’re accustomed to.
But how do you get the average user, for whom the cost of licensing the OS is completely opaque, to even think about cost at all? The computer they bought comes with Windows or MacOS on it already. Neither of which currently has any additional recurring monetary cost to the user.
You’d need mass-market laptops and desktops coming with a Linux distribution tuned well enough to run Microsoft Office and Adobe products without any more work for the user than running them on Windows. It needs to come pre installed and work so well at the “prosumer” use cases that they aren’t constantly thinking about how much easier it was to run Windows. Doing that means the OEM has to do much more unit testing and compatibility checks to ensure that when the customer opens the box and goes to install Steam and Apex or whatever that it just works without any terminal work necessary. Add to that that the OEM will want support from the company that manages the OS, and suddenly the cost to license tried and true Windows vs almost any Linux distribution for end user workstations is nearly moot.
And to make a dent in gaming, there is still an ocean to cross in terms of driver readiness and ease of use. It’s coming along, no doubt, and Valve investing as heavily as they are in Linux gaming is sure to move the needle, but it will still be an area of difficulty for some time because the user experience needs to accommodate completely custom builds with unexpected hardware configurations and box-built gaming PCs that can be OE tested and configured and everything in between.