Because the Steam UI is limited to their software, you would still need a desktop environment and they chose KDE Plasma, if you want you could can customize it to look like anything or just replace it with Gnome (or other desktop environment for that you fancy). My guess to why they would not do this is you would create a dependency on you as a company to apply/check the changes for every update, instead of just relying on the desktop supplier (KDE) if you use the default UI.

Which it is, it is a fully fledged OS that runs steam in big picture mode when you start it. It also says on the page for this device that if you want you can install any OS onto it.

Yes, Steam Machine is optimized for gaming, but it’s still your PC. Install your own apps, or even another operating system. Who are we to tell you how to use your computer? - https://store.steampowered.com/sale/steammachine

Winboat could be stopgap measure until you no longer need that software

That depends on where you fall on this chart

/s