If liability... what "might happen" where people would sue the city... is your only argument... Then every hiking trail on Oahu should be closed. "Oh dear, what if a tourist slipped and fell while climbing the Diamond Head Trail... or the Koko Head Stairs?"  Yes, it is a stupid idea to close all the hiking trails, especially the Haiku Stairs which has a very good safety record compared to all the other hiking trails on Oahu.