The hunters sent into the holes so far have not returned...
Who says that they could not return?
Maybe they simply do not WANT to return.
Jin's strength is an exception in his "own world" but there are other S ranked hunters annd people as strong as them.
So that strength is not "unknown".
But in that "world beyond the hole" slaying a dragon that Jin considers "small" ( as such maybe not even a S ranked but only an A ranked monster) is already unimaginable to the inhabitants there.
Therefore a hunter from Jin's "world" would probably considered far stronger in that other "world".
E.g.: An average hunter in Jin's "world" might be considered a veteran in the other "world", a veteran hunter in Jin's "world" might be considered an elite in the other "world".
If so then the hunters who went down the hole might simply not want to return because earning a living in the other "world" might be far easier and as such they can live more comfortably there.
Has anyone ever thought about that?
Of course, this is not something the people of Jin's "world" could know cause they do not know what lies beyond the holes.
But Jin could make such an assumption and any reader could think about it, too.
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Also one more point:
I have written it as "world" everytime because there is another possibility other than it being "2 worlds".
Namely those holes could also be connections through time.
So instead of "2 worlds" it could also be "2 ages".
After all we have just learned in this chapter that the dragon Zeld'don is/was a monster mentioned in the ANCIENT TIMES.
So that would implicate that Jin travelled through time instead of dimensions.
It could also explain why the strength of the people is not comparable to Jin's "time age" because in the ancient times the people were not as organized with hunters and experienced with monsters as they are several ages later.
Hence people dying before becoming strong enough to fight those strong monsters on their own would be more likely because there is no organisation in the ancient times like the hunters guild in Jin's age that regulates targets to be given out to fighters with the needed strength for the requests.