For many years we measured how usable Linux was based on its use as a desktop PC, but what as computers swap from desktop PC to smartphones, tablets etc.

I would argue that Linux has become the ultimate user-friendly operating system, in the sense that most users are unaware of its use. Most home wifi routers are running Linux and so is every Android mobile phone or tablet

Yes you can use Linux on the desktop, but most of what you are actually using it’s Linux, it’s a whole suite of Open Source sofware. Your desktop enviomnent might be Gnome or KDE for example. Similarly, Android uses Linux to deal with all the interactions with the hardware, but they layer over the top of that an interface designed to meet the needs of a phone.