What? You DO realize the original SD version would ALSO be pillarboxed, right??
Might as well have better resolution, but unless you want the show stretched or for the top and bottom to be cut off, there IS no other option besides drawing new animation for the pillarboxes, which may be possible for older shows but was NOT done for Outlaw Star!
So keeping it SD would fix none of that. TVs were largely still square in the late '90s and have just recently been wider, and rectangular, mainly because theatrical movies were forced to lose the sides when they aired on TV, and because of that, it evolved that TVs be elongated just as theatrical movies are.
Of course, the unfortunate side-effect is that older shows don't quite fit the new TV width, which is why we have pillarboxing.