Ah damn, thats what you get for doing something in a hurry. But the point stands, 16-bit is more than enough for what you can display with today's hardware.
16-bit images can already have 4294967296 different shades of color per channel. 64-bit image would have 18446744073709551616 per channel, and as most monitors can't really display all the different colors of 16-bit images, 64-bit images would really just be a waste of disk space at the moment. Maybe when monitors have 1…