Why does the Blu-ray specification only allow for a fixed set of resolutions instead of defining an upper bound on resolution? If arbitrary resolutions were allowed it would solve two problems:
Double letter boxing or window-boxing. This happens when a movie has hard borders and the aspect ratio of the viewing device does not match the aspect ratio of the video.
Certain movies were mastered at resolutions not in the list of allowed resolutions. This would avoid the need to resample the video which either results in a reduction in resolution or having to store upscaled video which wastes space.
Blu-Ray players already need to upscale or downscale the video to match the resolution of the device it's plugged into so I don't see any reason for not allowing this.
If the reason for not doing that is complexity, what about a fixed horizontal resolution and an arbitrary vertical resolution with an upper bound? That should be very simple to do because the player won't need to change how it upscales or downscales video it would just need to center it.