So yes, the original frame was probably closer to 4x3, but the 16x9 version is a bit better (than the full screen version).
Here is Chamber of Secrets for an examle:
Full and Wide Screens (overlayed)
HD
As you can see on the whole only a bit of information is lost from the left and right.
Sure this probably isn't what the Director intended, but the composition doesn't seem to get as far off as a full frame version.