Children don't want to fall down stairs.
They do, however, want to move in the direction of the stairs, and you cannot "help them not fall down stairs" without forcing them not to move in the direction of the stairs.
You are trying to reject a philosophy based on edge cases without trying to understand the big problems the philosophy is trying to solve.
Let's give some context to the stair-falling scenario. Consider that the parent is a TCS parent, not a normie parent. This parent has in fact heard the stair-falling scenario many times. It is often the first thing other people bring up when TCS is discussed.
Given the TCS parent has in fact thought about stair falling way more than a normie parent, how do you think the TCS parent has set up their home? Is it going to be ...