What wears out on a wooden stair
On an older wooden stair the handrail takes the abuse. Hands rub the stain thin where you grab it, a baluster works loose from years of bumps, and the newel at the bottom starts to wobble when someone leans on it. Those signs show up long before the treads give out.
A cracked or wobbling newel is the one to deal with early, because the whole rail ties into it. B.R. Baumann said Marlon explained the process and what the options would be, so you know which parts are sound and which have to come out before the work starts.