@emilygorcenski Not falling apart is sort of step zero, yeah. So no needing to make a rapid trip to Lowe's trip for jack posts which I was wondering about from the picture.
I will admit to being a bit baffled as to how stringers would ever get that way but after seeing with my own eyes where someone notched nominal 2x12 joists about 10" deep for pipe I should be able to say "old house" to myself and acknowledge that "why" is a frequently unanswerable question.