Will not happen. You posted in the lore part, so you most likely know that the heroes actually defeating all those threads are most of the time different ones.
We as the players are mostly just playing a smaller role to allow the important ones to finish their job (in lore, that is).
Long story short: no rest for the wicked. Even in MoP we extinguished many threats just before they were able to actually do any harm (similar to most of classic WoW)