Public buildings have to account for amenities, toilets per head, actual physical space in the building for safe movement of people in normal operation, add extra for unsafe situations.
The upper walk way acts as an emergency route, it does not need exits on it, but allows max capacity if lower routes become blocked. That is why the photo depicts a much wider space, to account for the seating, IAW has not factored seating into that area, because the arena has not factored extra capacity into the design.
The metal work of the structure was not factored for added load that would be required to increase capacity, raise roof etc etc.
It's an arena for what it was designed for. The current capacity I'm afraid is all your going to get.
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