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Fair enough. However flight attendants only ask the people who sit in the side seat blocks if they are capable of assisting with opening the door. That means that theoretically someone who was infirm or moved slowly could end up sitting in the middle block and therefore delay or prevent the kind of evacuation you mention.
A child (and perhaps an elderly person) would only be blocked from sitting there because their ticket details indicate their status but someone who would be disqualified from the side block but was otherwise identically ticketed to a standard adult would not be blocked from the middle section.