You're missing the point, I think. IMO, it is unequivocal that in theory continuing to operate the site for some time could lead to substantially more arrests. Whether or not the FBI succeeded in doing so is an operational matter. If they failed to exploit this opportunity properly, that doesn't invalidate the concept.

Now, you could criticize their operational use of this power, but that is entirely distinct from criticizing, in general, the use of this sort of tactic. Which, I think, on its face, is clear can be an effective one.