There was another reason why Jin wanted Yun to be the one to enter the 'experimental instance' alongside the cultivators. It indeed would be an experiment, though not the kind his customers had expected.
Though the System might appear almighty, Jin had noticed in his fight against the Ruby Rats that its powers did have certain limitations. For example, it could create a sheer limitless number of clones for regular minions (it wasn't entirely free, they cost souls), but the System could only exercise such powers inside of dimensional instances and the areas it owned.
For this reason, during the cultivators' participation in the Goblin World, the monsters that had accompanied them had all been the originals. Given the sheer number of customers, even his bellators had to help out acting as guides once in a while, or the monsters cut short their interactions. After Jin had calibrated the actual Goblin Raid dimensional instances, this was no longer an issue.
