Unless you're ready now for a full redesign of the staircase, maybe have it painted to match the woodwork? (Same for the white trim strip on the left side of the stairs.) A good painter can make it look like the staircase wood (better than I did here!).

You could also just box it in with drywall and paint it the wall color so it looks more like the circled wall in the background

You could probably replace it with 2" square tube steel.