Hearing the story of how the Golden City of Origin fell, everyone listening to it fell silent. Hundreds of billions of bugs were too much to handle for almost everyone here. They might be able to kill a few millions of them but soon they would be completely buried by that amount of bugs sooner or later.
And it was not like all the bugs were weak. Obviously amongst the billions of bugs, most of them were super weak and could be killed in one blow. But still there were tens of thousands of other bugs who were almost as strong as an UR grade or even stronger. Without them destroying the Golden City was impossible.
Obviously the Nest of All Chaos was also badly hurt after this epic fight. But it's been thousands of years since then. It would be foolish to believe that they were still suffering from that battle. Rein and his group needed to assume that the Nest of All Chaos was now even stronger than that time and they needed to come up with a plan based on that assumption.
