We can replace the operating system of an Android phone when we have the necessary hardware drivers. Using NixOS as the operating system allows us to reliably recompile most of the modern software that we rely on for the mobile operating system to get the benefits of using real applications instead of just the limited capabilities of mobile apps. For example, programs like databases, webservices, CAD, compilers, image processing can run natively on the phone when you get rid of the Android barrier that makes these impossible to implement without a total rewrite. The size of the screen is not the problem because you can cast it to a monitor, and useful embedded computers often don't have their own screens.