There are multiple seats per row and only the first person of that row may benefit from the concurrency, as each person gets into the isle for that row they wil

There are multiple seats per row and only the first person of that row may benefit from the concurrency, as each person gets into the isle for that row they will add those seconds and that is sequential not concurrent. Also you likely haven't been in a trampling crowd situation, bag on a chest doesn't occupy zero space.