^^ I don't know. I think that while this cycle may have happened before of finding replacements. Jacob may have been a replacement for someone/thing. This seems like it's the end of the cycle. Clearly MiB's goal is to end the cycle at least by killing all of the candidates off. Doesn't mean it will happen.
There also seems to be the implication that even though Jacob is dead, the candidate selection process can continue and someone can still be selected. Something knows that there are still candidates alive and hence MiB cannot get off the island. Apparently the test runs independentally of Jacob.
I guess we'll see if the cycle ends or there is a replacement. Agree that it seems like a big test.