16:9 versions of "Seinfeld" are already being circulated. They're cropped, so in fact there is less
image, not more. I can't speak for how they shot that series though, maybe the film negatives don't show unintended details like the TNG ones?
Here's another one, from "Q Who":
This is a full frame of unexposed film, from which a 16:9 version could
be printed... the white box represents the 4:3 frame, the image that actually appears in the episode. As you can all see, the decals on the saucer are not complete, because the people involved in shooting the scene never intended for these details of the saucer to be seen. All that mattered was what was inside the white box section. That
section is fine, but a widescreen copy would make it obvious that the Enterprise is only a model! This
is why the 4:3 format has been maintained.