This doesn’t make much sense.
Seems to me like humans have full control over the dungeon now, if you kill the boss regularly, then there would be zero problems? Our MC was able to solo kill a 30year grown boss, so how easy would a new boss be?
Just keep the town a bit away from the dungeon entrance with defenses and all is good? I don’t really see the problem.
The biggest issue was the uncontrolled growth and being way too close to the dungeon for surprise outbreaks, without buffer zones, but that can be handled easily by good management and regular purges.
You know the thing that hunters do for our forests?
Weird that nobody here in the comments talked about it and instead just cried over the merchant talking BS.
Tl;dr nothing that the author wrote, convinced me that with the fizzy potions this dungeon was any problem after downing the overgrown boss. If people used their brain there is no reason to seal it at the moment, expect for emotional reasons.
P.S. in many other Novels/Mangas dungeon management and things like regular purges and co. are mentioned.
Was this just written to add drama and give the twins the stage to come up with a new idea? They did talk/think about monsters reproducing naturally after all.