One thing about changing lyrics is that most people don't have that many hymns memorized & if they do it's usually just the first verse. People will notice changing 'O man' the rest would probably pass unnoticed by most. I briefly thought about 'O Earth' for a more universal cast to the hymn, but given the 2nd line just would not fly. The word Man in the original can really be interpreted as either all people, or generic individual person. Choosing another word does change that, perhaps deliberate, ambiguity.

Changing the grammar to be less archaic is much more noticable even when you don't know quite what it is that's going on. Given that these words come from 1919, the grammar was already outdated as compared to normal spoken English, so staying with the author's original words fits.