There is good reason it can't.
The logic to download content, set schedules, check files are complete etc etc are far too complex for something that runs in a browser alone. There's also push messaging to handle, interaction with serial ports, HDMI-CEC, shell commands, all stuff that a browser can't directly do.
I'm not sure what you mean by Microsoft browser licencing issue. If you were to run Xibo in a browser (if that existed), then the end user would need to licence Windows with Microsoft, and hence Internet Explorer. The project wouldn't bear any of that burden.
The preview mode will accurately render what you would see in the Android Player, and what we hope to deliver in the next major version of the Windows Player, so I don't believe it to be "useless".